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/rmcp-client/tests/streamable_http_test_support.rs

@@ -98,6 +98,7 @@ pub(crate) async fn create_client(base_url: &str) -> anyhow::Result<RmcpClient>
         /*env_http_headers*/ None,
         OAuthCredentialsStoreMode::File,
         Environment::default_for_tests().get_http_client(),
+        /*auth_provider*/ None,
     )
     .await?;
 
@@ -135,6 +136,7 @@ pub(crate) async fn create_remote_client(
         /*env_http_headers*/ None,
         OAuthCredentialsStoreMode::File,
         Arc::new(http_client),
+        /*auth_provider*/ None,
     )
     .await?;