The stream consumer called prefers_fresh_final_streaming(text,
metadata=...) only, and no metadata producer stamps a platform key — so
RelayAdapter's hook always fell back to the PRIMARY descriptor's
platform (the scalar-vs-per-chat capability seam, third occurrence).
Two failure directions on multiplexed relays with
platforms.relay.extra.slack.unfurl_links/media: true (#97957):
- Slack primary fronting Telegram/Discord: every link-bearing streamed
final on the non-Slack chats finalized as a fresh send with no delete
op advertised -> the answer delivered TWICE (orphaned preview).
- Non-Slack primary fronting Slack: the hook returned False, leaving
the force-on unfurl feature dark on exactly the chats it shipped for.
Pass chat_id=self.chat_id from the consumer; the relay hook already
accepted it and resolves via _platform_by_chat + the per-platform
negotiated descriptor. Graduated TypeError fallback keeps the
single-platform hook signatures (Telegram, base class) and legacy test
doubles working unchanged.
Both regression tests verified RED against the unfixed consumer, GREEN
with the fix; single-platform relays are unaffected (#97957's own 30
tests unchanged-green).
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