## 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
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