Mobile: subsequent agent replies in a thread attach to the original post

## Summary

In the Buzz mobile app, an agent's first reply in a thread works, but subsequent in-thread calls to the agent do not reply to the newer message. The agent reply is attached to the original top-level chat post instead.

## Steps to reproduce

1. In the mobile app, create a channel message and mention an agent.
2. Let the agent reply in the resulting thread.
3. Add another reply in that same thread and mention/call the agent again.
4. Observe where the agent's response is attached.

## Actual behavior

The subsequent agent response replies to the first/top-level chat post rather than the latest in-thread message.

## Expected behavior

The agent response should use the current in-thread message as its reply target, preserving the conversation's nesting/context.

## Impact

Follow-up conversations with agents on mobile lose their correct reply context, making threaded exchanges difficult to follow.