Top-level channel messages from the same sender shared a bare channel
debounce key, causing concurrent messages in different threads to merge
into a single reply on the wrong thread. Now the debounce key includes
the message timestamp for top-level messages, matching how the downstream
session layer already scopes by canonicalThreadId.
Extracted buildSlackDebounceKey() for testability.
Closes#31935
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
