refactor: route Codex auth through AuthProvider (#18811)

## Summary

This PR moves Codex backend request authentication from direct
bearer-token handling to `AuthProvider`.

The new `codex-auth-provider` crate defines the shared request-auth
trait. `CodexAuth::provider()` returns a provider that can apply all
headers needed for the selected auth mode.

This lets ChatGPT token auth and AgentIdentity auth share the same
callsite path:
- ChatGPT token auth applies bearer auth plus account/FedRAMP headers
where needed.
- AgentIdentity auth applies AgentAssertion plus account/FedRAMP headers
where needed.

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

## Callsite Migration

| Area | Change |
| --- | --- |
| backend-client | accepts an `AuthProvider` instead of a raw
token/header |
| chatgpt client/connectors | applies auth through
`CodexAuth::provider()` |
| cloud tasks | keeps Codex-backend gating, applies auth through
provider |
| cloud requirements | uses Codex-backend auth checks and provider
headers |
| app-server remote control | applies provider headers for backend calls
|
| MCP Apps/connectors | gates on `uses_codex_backend()` and keys caches
from generic account getters |
| model refresh | treats AgentIdentity as Codex-backend auth |
| OpenAI file upload path | rejects non-Codex-backend auth before
applying headers |
| core client setup | keeps model-provider auth flow and allows
AgentIdentity through provider-backed OpenAI 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. https://github.com/openai/codex/pull/18785: explicit AgentIdentity
auth mode and startup task allocation
4. This PR: 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/cloud-tasks/src/lib.rs

@@ -68,7 +68,7 @@ async fn init_backend(user_agent_suffix: &str) -> anyhow::Result<BackendContext>
     };
     append_error_log(format!("startup: base_url={base_url} path_style={style}"));
 
-    let auth_manager = util::load_auth_manager().await;
+    let auth_manager = util::load_auth_manager(Some(base_url.clone())).await;
     let auth = match auth_manager.as_ref() {
         Some(manager) => manager.auth().await,
         None => None,
@@ -87,23 +87,17 @@ async fn init_backend(user_agent_suffix: &str) -> anyhow::Result<BackendContext>
         append_error_log(format!("auth: mode=ChatGPT account_id={acc}"));
     }
 
-    let token = match auth.get_token() {
-        Ok(t) if !t.is_empty() => t,
-        _ => {
-            eprintln!(
-                "Not signed in. Please run 'codex login' to sign in with ChatGPT, then re-run 'codex cloud'."
-            );
-            std::process::exit(1);
-        }
-    };
+    if !auth.uses_codex_backend() {
+        eprintln!(
+            "Not signed in. Please run 'codex login' to sign in with ChatGPT, then re-run 'codex cloud'."
+        );
+        std::process::exit(1);
+    }
 
-    http = http.with_bearer_token(token.clone());
-    if let Some(acc) = auth
-        .get_account_id()
-        .or_else(|| util::extract_chatgpt_account_id(&token))
-    {
+    let auth_provider = codex_model_provider::auth_provider_from_auth(&auth);
+    http = http.with_auth_provider(auth_provider);
+    if let Some(acc) = auth.get_account_id() {
         append_error_log(format!("auth: set ChatGPT-Account-Id header: {acc}"));
-        http = http.with_chatgpt_account_id(acc);
     }
 
     Ok(BackendContext {