Avoid blocking thread startup on MCP OAuth discovery (#32825)

## Why

OAuth metadata discovery for an optional HTTP MCP server can stall, delaying
`thread/start` even though that server is not required.

## What changed

- Remove eager MCP authentication-status discovery from session and connection
  manager startup.
- When an MCP connection reports an authentication error, inspect configured and
  stored credentials locally to preserve login and reauthentication failure
  reasons without making another discovery request.
- Continue using server configuration directly for startup error messages and
  timeout hints.

## Testing

Add an app-server regression test with an optional HTTP MCP endpoint that accepts
a connection but never responds, and verify that `thread/start` completes without
waiting for its configured startup timeout.

GitOrigin-RevId: 56fe0188cc1ceaf746b0b65e8b7dd1fd2e13420d

codex-rs/codex-mcp/src/mcp/mod.rs

@@ -310,14 +310,6 @@ pub async fn read_mcp_resource(
 ) -> anyhow::Result<ReadResourceResult> {
     let mut mcp_servers = effective_mcp_servers(config, auth);
     mcp_servers.retain(|name, _| name == server);
-    let auth_statuses = compute_auth_statuses(
-        mcp_servers.iter(),
-        config.mcp_oauth_credentials_store_mode,
-        config.auth_keyring_backend_kind,
-        auth,
-        &runtime_context,
-    )
-    .await;
     let (tx_event, rx_event) = unbounded();
     drop(rx_event);
     let cancel_token = CancellationToken::new();
@@ -325,7 +317,6 @@ pub async fn read_mcp_resource(
         &mcp_servers,
         config.mcp_oauth_credentials_store_mode,
         config.auth_keyring_backend_kind,
-        auth_statuses,
         &config.approval_policy,
         String::new(),
         tx_event,
@@ -404,7 +395,6 @@ pub async fn collect_mcp_server_status_snapshot_with_detail(
         &mcp_servers,
         config.mcp_oauth_credentials_store_mode,
         config.auth_keyring_backend_kind,
-        auth_status_entries.clone(),
         &config.approval_policy,
         submit_id,
         tx_event,