Route extension warnings to app-server threads (#35168)

## Why

Extension warnings need a stable thread target even when they are emitted outside an active turn.

## What changed

- Add `ExtensionWarning` and a dedicated `ExtensionEventSink::emit_warning` path with an explicit thread ID and optional turn ID.
- Deliver extension warnings as thread-scoped app-server `warning` notifications, preserving listener ordering and waiting briefly for a subscriber when no listener is active.
- Move skills warnings to the new path, limiting provider catalog warnings to four messages of 256 bytes each.

## Testing

- Cover warning routing, ordering, UTF-8-safe truncation, subscriber fallback, and invalid thread IDs.
- Verify skills warning bounds and app-server delivery of executor catalog budget warnings.

GitOrigin-RevId: 4dcb1ba7f17c0edb1a9d548f9568e0c734bffc15

codex-rs/ext/extension-api/src/lib.rs

@@ -7,6 +7,7 @@ mod user_instructions;
 pub use capabilities::AgentSpawnFuture;
 pub use capabilities::AgentSpawner;
 pub use capabilities::ExtensionEventSink;
+pub use capabilities::ExtensionWarning;
 pub use capabilities::NoopExtensionEventSink;
 pub use capabilities::NoopResponseItemInjector;
 pub use capabilities::ResponseItemInjectionFuture;