Prewarm MCP runtime updates in the background (#35144)

## Why

Changes to MCP inputs can require rebuilding the runtime before the next model
step. Start that work eagerly so the step is less likely to wait for it.

## What changed

- Add a per-session worker that prewarms dirty MCP state after configuration,
  authentication, or explicit refresh changes.
- Coalesce queued refreshes and always rebuild from the latest session state,
  while keeping exact model-step refreshes as the correctness path.
- Stop and join the worker during session shutdown.

## Testing

- Verify rapid refreshes skip superseded configuration and publish the latest
  state.
- Verify MCP-relevant policy changes mark the runtime for refresh.

GitOrigin-RevId: 045ed406caff97dcbb483bc183fd16eae0c7d7ad

codex-rs/core/src/session/mcp.rs

@@ -502,7 +502,7 @@ impl Session {
         self.services
             .supports_openai_form_elicitation
             .store(supported, std::sync::atomic::Ordering::Relaxed);
-        self.mark_mcp_runtime_dirty();
+        self.request_mcp_runtime_refresh();
         Ok(())
     }