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- Verify rapid refreshes skip superseded configuration and publish the latest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/tools/spec_plan_tests.rs

@@ -1119,6 +1119,7 @@ async fn tool_search_cache_rebuilds_when_deferred_world_state_changes() {
                 )],
                 tool_suggest_candidates: None,
                 extension_tool_executors: Vec::new(),
+                wait_for_environment_tool_config: None,
                 dynamic_tools: &[],
             },
             &cache,