[login] revoke existing auth before starting login (#27674)

## Why

`codex login` previously persisted newly issued OAuth credentials and
only then attempted to revoke the superseded refresh token. The old
credential must be revoked before a replacement browser or device-code
flow starts, and successful login must not perform any post-login
revocation attempt.

## What changed

- Revoke and clear existing stored auth before browser or device-code
CLI login begins.
- Remove superseded-token detection and revocation from the shared token
persistence path; successful login now only saves the new credentials.
- Read the raw configured auth store during CLI cleanup so
environment-provided auth cannot mask the stored refresh token.
- Preserve `auto` storage fallback semantics when keyring deletion fails
by clearing the fallback auth file.
- Add a process-level CLI regression test that requires the revoke
request to precede every device-login request and occur exactly once.

If replacement login is canceled or fails, the previous local
credentials have already been cleared. Remote revocation remains best
effort, matching explicit logout behavior.

## Validation

### Process-level before/after reproduction

I compiled the real `codex` CLI from the pre-fix parent (`14df0e8833`)
and from the PR implementation (`25c002f23b`; the login behavior is
unchanged at the current head), then ran the same device-code flow
against a local HTTP mock OAuth authority.

Each run:

1. Used a fresh temporary `CODEX_HOME` configured with
`cli_auth_credentials_store = "file"`.
2. Seeded that temporary home with managed ChatGPT auth containing
`old-access` and `old-refresh` tokens.
3. Pointed `CODEX_REVOKE_TOKEN_URL_OVERRIDE` at the mock `/oauth/revoke`
endpoint.
4. Ran the compiled CLI as:

   ```shell
   CODEX_HOME=<temporary-home> \
     CODEX_REVOKE_TOKEN_URL_OVERRIDE=<mock-issuer>/oauth/revoke \
<compiled-codex> login --device-auth --experimental_issuer <mock-issuer>
   ```

5. Recorded every request received by the mock authority. The mock
marked `new-access` valid when `/oauth/token` issued it and invalidated
it if `/oauth/revoke` arrived afterward, reproducing the observed
session-invalidating failure mode. After login exited, the harness also
verified the persisted refresh token and probed a protected endpoint
with `new-access`.

| Build | Observed request order | CLI/persistence result | `new-access`
probe |
| --- | --- | --- | --- |
| Pre-fix | `usercode → device token → OAuth token →
revoke(old-refresh)` | Exit `0`; `new-refresh` persisted | `401` |
| PR | `revoke(old-refresh) → usercode → device token → OAuth token` |
Exit `0`; `new-refresh` persisted | `200` |

The PR run therefore issued exactly one revocation request, before any
request that initiated the replacement login, and issued no revocation
after token exchange.

### Regression coverage


`codex-rs/cli/tests/login.rs::device_login_revokes_existing_auth_before_requesting_new_tokens`
runs the real first-party `codex` binary against a `wiremock` OAuth
server with an isolated temporary `CODEX_HOME`. It asserts:

- the exact request sequence is `/oauth/revoke`,
`/api/accounts/deviceauth/usercode`, `/api/accounts/deviceauth/token`,
then `/oauth/token`;
- there is exactly one revoke request and its body contains
`old-refresh` with the `refresh_token` hint;
- the completed login persists `new-refresh`.

Local validation:

- `just test -p codex-login` — 130 passed
- `just test -p codex-cli` — 280 passed, including the new process-level
regression test
- `just bazel-lock-check`

codex-rs/login/src/auth/manager.rs

@@ -590,15 +590,17 @@ pub async fn logout_with_revoke(
     codex_home: &Path,
     auth_credentials_store_mode: AuthCredentialsStoreMode,
 ) -> std::io::Result<bool> {
-    AuthManager::new(
-        codex_home.to_path_buf(),
-        /*enable_codex_api_key_env*/ false,
-        auth_credentials_store_mode,
-        /*chatgpt_base_url*/ None,
-    )
-    .await
-    .logout_with_revoke()
-    .await
+    let auth_dot_json = match load_auth_dot_json(codex_home, auth_credentials_store_mode) {
+        Ok(auth_dot_json) => auth_dot_json,
+        Err(err) => {
+            tracing::warn!("failed to load stored auth during logout: {err}");
+            None
+        }
+    };
+    if let Err(err) = revoke_auth_tokens(auth_dot_json.as_ref()).await {
+        tracing::warn!("failed to revoke auth tokens during logout: {err}");
+    }
+    logout_all_stores(codex_home, auth_credentials_store_mode)
 }
 
 /// Writes an `auth.json` that contains only the API key.