Route MCP auth discovery through runtime HTTP clients (#35239)

## Why

MCP authentication checks need to use the same HTTP routing as the MCP
transport so servers reached through configured proxies can be discovered
reliably.

## What changed

- Resolve OAuth discovery and authentication status through each server's
  runtime HTTP client for both local and managed environments.
- Keep local discovery capped at five seconds while allowing explicit login
  requests to retain their requested timeout.
- Resolve refreshed MCP configuration and its runtime context from the same
  snapshot.

## Testing

- Cover OAuth discovery through an environment proxy and macOS system proxy
  resolution.
- Cover proxied MCP startup and runtime refresh with updated authorization
  headers.
- Verify capped and preserved OAuth discovery timeout policies.

GitOrigin-RevId: 461fb1d4786e547df8b1e6b2215a8ac40438a3aa

codex-rs/rmcp-client/src/lib.rs

@@ -15,6 +15,7 @@ mod utils;
 
 pub use auth_status::McpAuthState;
 pub use auth_status::McpLoginRequirement;
+pub use auth_status::OAuthDiscoveryTimeout;
 pub use auth_status::StreamableHttpOAuthDiscovery;
 pub use auth_status::determine_streamable_http_auth_status;
 pub use auth_status::determine_streamable_http_auth_status_from_credentials;