Clean up completed response controllers

[cubrift]

Problem

messages/ai/MessageAI.js stores an AbortController in activeGenerations for each chat, but completed generations are not consistently removed. The map is cleaned up when a newer message arrives or when a group message is ignored, but successful text responses and several early-return paths can leave controllers retained indefinitely.

Expected behavior

The map should track only generations that are still running. A completed or failed generation should release its controller without deleting a newer generation that replaced it.

Suggested direction

Use a guarded cleanup path, such as a finally block or a small helper that checks whether the stored controller is the same controller being completed.

Acceptance criteria

• Successful text responses remove their controller.
• Audio, aborted, ignored, and error paths do not leave stale controllers behind.
• Cleanup never removes a newer controller for the same chat.
• A focused test or reproducible verification demonstrates the map is empty after a completed generation.

[cubrift]

Every response occurs in sequence, and every AbortController is redefined for each one. Leaving controllers retained do not matter and this isn't considered as an issue right now.