fix(#701): persist thread messages before switching to new chat

[shogun444]

Closes #701

What

This PR addresses the message persistence issue when switching to a new chat by introducing an optional cacheMessages method to ThreadStorage.

The default in-memory storage implements this method by caching the current messages before the local state is cleared, allowing completed conversations to be restored when switching back to the thread.

Changes

• Add optional cacheMessages(threadId, messages) to ThreadStorage
• Implement cacheMessages in the default in-memory storage
• Cache current messages before switchToNewThread() clears the local state
• Persist messages after successful LLM streaming completes
• Add unit tests covering both persistence paths

Note: While validating this change, I noticed another reproducible issue when switching threads during an active streaming response. I've documented that separately in the issue discussion since it appears to be a different edge case.

Test Plan

• Verified locally
• Added unit tests covering both persistence paths

Checklist

• I linked a related issue
• I updated docs/README when needed (not required)
• I considered backwards compatibility

[shogun444]

While testing this a bit more, I noticed another case that may be related.

If I send a message and the assistant starts streaming a response (for example, generating a component), then quickly switch to another chat or click New Chat before the response finishes, and later come back to the original thread, the message I originally sent is sometimes gone as well.

From what I've seen:

• Start a new conversation.
• Send a prompt that produces a longer streamed response.
• While it's still streaming, switch to another thread or click New Chat.
• Return to the original thread.

Actual:

• In some cases, the user message itself disappears from the conversation.

I'm not sure if this is the same underlying issue or a separate race condition, but I was able to reproduce it multiple times while testing.