fix(chat-saga/chat-state): handle race condition in conversation validation by cancelling previous tasks

[domw30]

What does this do?

• Implements a task cancellation mechanism in the conversation validation process to prevent race conditions when users navigate between conversations quickly.

Why are we making this change?

• To fix a bug where the active conversation ID could be incorrectly set when a user navigates away from a conversation before validation completes, causing UI inconsistencies between the URL and displayed conversation.

How do I test this?

• run tests as usual
• run UI and navigate between channels and messenger app to test correct room content is rendered

Key decisions and Risk Assessment:

Things to consider:

1. How will this affect security?
2. How will this affect performance?
3. Does this change any APIs?