Populate `reply_to_message_id`, `reply_to_text`, and
`reply_to_is_own_message` on reaction events so the gateway injects
`[Replying to your previous message: "..."]` when the agent receives
a tapback.
The sidecar now extracts a capped text preview from the hydrated
reaction target (plain text and mixed group messages; null for
attachment/voice-only targets), emitting it as `targetText` in the
NDJSON reaction payload. The Python adapter reads this field and sets
the reply correlation fields on the `MessageEvent`.
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