feat: add explicit AgentIdentity auth mode (#18785)

## Summary

This PR adds `CodexAuth::AgentIdentity` as an explicit auth mode.

An AgentIdentity auth record is a standalone `auth.json` mode. When
`AuthManager::auth().await` loads that mode, it registers one
process-scoped task and stores it in runtime-only state on the auth
value. Header creation stays synchronous after that because the task is
initialized before callers receive the auth object.

This PR also removes the old feature flag path. AgentIdentity is
selected by explicit auth mode, not by a hidden flag or lazy mutation of
ChatGPT auth records.

Reference old stack: https://github.com/openai/codex/pull/17387/changes

## Design Decisions

- AgentIdentity is a real auth enum variant because it can be the only
credential in `auth.json`.
- The process task is ephemeral runtime state. It is not serialized and
is not stored in rollout/session data.
- Account/user metadata needed by existing Codex backend checks lives on
the AgentIdentity record for now.
- `is_chatgpt_auth()` remains token-specific.
- `uses_codex_backend()` is the broader predicate for ChatGPT-token auth
and AgentIdentity auth.

## Stack

1. https://github.com/openai/codex/pull/18757: full revert
2. https://github.com/openai/codex/pull/18871: isolated Agent Identity
crate
3. This PR: explicit AgentIdentity auth mode and startup task allocation
4. https://github.com/openai/codex/pull/18811: migrate Codex backend
auth callsites through AuthProvider
5. https://github.com/openai/codex/pull/18904: accept AgentIdentity JWTs
and load `CODEX_AGENT_IDENTITY`

## Testing

Tests: targeted Rust checks, cargo-shear, Bazel lock check, and CI.

codex-rs/login/tests/suite/auth_refresh.rs

@@ -54,6 +54,7 @@ async fn refresh_token_succeeds_updates_storage() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -117,6 +118,7 @@ async fn refresh_token_refreshes_when_auth_is_unchanged() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -171,6 +173,7 @@ async fn refresh_token_skips_refresh_when_auth_changed() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -180,6 +183,7 @@ async fn refresh_token_skips_refresh_when_auth_changed() -> Result<()> {
         openai_api_key: None,
         tokens: Some(disk_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     save_auth(
         ctx.codex_home.path(),
@@ -234,6 +238,7 @@ async fn refresh_token_errors_on_account_mismatch() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -244,6 +249,7 @@ async fn refresh_token_errors_on_account_mismatch() -> Result<()> {
         openai_api_key: None,
         tokens: Some(disk_tokens),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     save_auth(
         ctx.codex_home.path(),
@@ -302,6 +308,7 @@ async fn returns_fresh_tokens_as_is() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(stale_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -349,6 +356,7 @@ async fn refreshes_token_when_access_token_is_expired() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(fresh_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -398,6 +406,7 @@ async fn auth_reloads_disk_auth_when_cached_auth_is_stale() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens),
         last_refresh: Some(stale_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -408,6 +417,7 @@ async fn auth_reloads_disk_auth_when_cached_auth_is_stale() -> Result<()> {
         openai_api_key: None,
         tokens: Some(disk_tokens.clone()),
         last_refresh: Some(fresh_refresh),
+        agent_identity: None,
     };
     save_auth(
         ctx.codex_home.path(),
@@ -459,6 +469,7 @@ async fn auth_reloads_disk_auth_without_calling_expired_refresh_token() -> Resul
         openai_api_key: None,
         tokens: Some(initial_tokens),
         last_refresh: Some(stale_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -469,6 +480,7 @@ async fn auth_reloads_disk_auth_without_calling_expired_refresh_token() -> Resul
         openai_api_key: None,
         tokens: Some(disk_tokens.clone()),
         last_refresh: Some(fresh_refresh),
+        agent_identity: None,
     };
     save_auth(
         ctx.codex_home.path(),
@@ -518,6 +530,7 @@ async fn refresh_token_returns_permanent_error_for_expired_refresh_token() -> Re
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -570,6 +583,7 @@ async fn refresh_token_does_not_retry_after_permanent_failure() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -636,6 +650,7 @@ async fn refresh_token_reloads_changed_auth_after_permanent_failure() -> Result<
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -657,6 +672,7 @@ async fn refresh_token_reloads_changed_auth_after_permanent_failure() -> Result<
         openai_api_key: None,
         tokens: Some(disk_tokens.clone()),
         last_refresh: Some(fresh_refresh),
+        agent_identity: None,
     };
     save_auth(
         ctx.codex_home.path(),
@@ -715,6 +731,7 @@ async fn refresh_token_returns_transient_error_on_server_failure() -> Result<()>
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -767,6 +784,7 @@ async fn unauthorized_recovery_reloads_then_refreshes_tokens() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -776,6 +794,7 @@ async fn unauthorized_recovery_reloads_then_refreshes_tokens() -> Result<()> {
         openai_api_key: None,
         tokens: Some(disk_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     save_auth(
         ctx.codex_home.path(),
@@ -859,6 +878,7 @@ async fn unauthorized_recovery_errors_on_account_mismatch() -> Result<()> {
         openai_api_key: None,
         tokens: Some(initial_tokens.clone()),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     ctx.write_auth(&initial_auth)?;
 
@@ -869,6 +889,7 @@ async fn unauthorized_recovery_errors_on_account_mismatch() -> Result<()> {
         openai_api_key: None,
         tokens: Some(disk_tokens),
         last_refresh: Some(initial_last_refresh),
+        agent_identity: None,
     };
     save_auth(
         ctx.codex_home.path(),
@@ -926,6 +947,7 @@ async fn unauthorized_recovery_requires_chatgpt_auth() -> Result<()> {
         openai_api_key: Some("sk-test".to_string()),
         tokens: None,
         last_refresh: None,
+        agent_identity: None,
     };
     ctx.write_auth(&auth)?;
 
@@ -962,6 +984,7 @@ impl RefreshTokenTestContext {
             codex_home.path().to_path_buf(),
             /*enable_codex_api_key_env*/ false,
             AuthCredentialsStoreMode::File,
+            /*chatgpt_base_url*/ None,
         );
 
         Ok(Self {

codex-rs/login/tests/suite/logout.rs

@@ -142,6 +142,7 @@ async fn auth_manager_logout_with_revoke_uses_cached_auth() -> Result<()> {
         codex_home.path().to_path_buf(),
         /*enable_codex_api_key_env*/ false,
         AuthCredentialsStoreMode::File,
+        /*chatgpt_base_url*/ None,
     );
     save_auth(
         codex_home.path(),
@@ -192,6 +193,7 @@ fn chatgpt_auth_with_refresh_token(refresh_token: &str) -> AuthDotJson {
             account_id: Some("account-id".to_string()),
         }),
         last_refresh: None,
+        agent_identity: None,
     }
 }