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/core/src/plugins/manager.rs

@@ -124,21 +124,11 @@ fn featured_plugin_ids_cache_key(
     config: &Config,
     auth: Option<&CodexAuth>,
 ) -> FeaturedPluginIdsCacheKey {
-    let token_data = auth.and_then(|auth| auth.get_token_data().ok());
-    let account_id = token_data
-        .as_ref()
-        .and_then(|token_data| token_data.account_id.clone());
-    let chatgpt_user_id = token_data
-        .as_ref()
-        .and_then(|token_data| token_data.id_token.chatgpt_user_id.clone());
-    let is_workspace_account = token_data
-        .as_ref()
-        .is_some_and(|token_data| token_data.id_token.is_workspace_account());
     FeaturedPluginIdsCacheKey {
         chatgpt_base_url: config.chatgpt_base_url.clone(),
-        account_id,
-        chatgpt_user_id,
-        is_workspace_account,
+        account_id: auth.and_then(CodexAuth::get_account_id),
+        chatgpt_user_id: auth.and_then(CodexAuth::get_chatgpt_user_id),
+        is_workspace_account: auth.is_some_and(CodexAuth::is_workspace_account),
     }
 }