join_voice_channel(channel, text_channel_id=..., source=...) is the programmatic join API, and it
kept the requested binding only on a cold join: the already-connected paths (same voice channel, or a
move) returned before writing it, so a successful join to another text channel kept transcribing
into the old one. All three paths now apply the binding through one helper. Moving to another text
channel drops speech still buffered for the old one, as /voice join does, and the old channel's bound
source unless a new one is given: its chat would keep routing turns into the old conversation.
With no bound source, a voice turn built a synthetic one with chat_type "channel". A typed message in
a guild text channel is "group" (or "thread" under its parent), so the voice turn keyed another
session. It now goes through build_source as a typed message does, with the guild, thread and parent.
/voice join, the only in-repo caller, already writes its binding itself after joining, so this is
latent until something calls join_voice_channel with a text channel (the open auto-join PRs would).
Reported by @andrexibiza as F7 in #132474.
a036b13793