merge_pending_message_event extends the existing event's media_urls in
place when two media-bearing messages arrive in quick succession (photo
bursts, consecutive voice messages). The gateway runner caches STT
transcripts on the event via _gateway_pending_stt_text; if the cached
event gains new media after the cache was populated, the stale transcript
was returned instead of transcribing the merged attachments.
Add _invalidate_pending_stt_cache() and call it from both media-merge
branches in merge_pending_message_event so the next transcription call
re-runs against the full merged media list.
Closes the merge-race edge case identified during review of PR #61519.
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