- Store source metadata on /voice channel join so voice input shares the
same session as the linked text channel conversation
- Treat voice-linked text channels as free-response (skip @mention and
auto-thread) while voice is active
- Scope the voice-linked exemption to the exact bound channel, not
sibling threads
- Guard signal handler registration in start_gateway() for non-main
threads (prevents RuntimeError when gateway runs in a daemon thread)
- Clean up _voice_sources on leave_voice_channel
Salvaged from PR #3475 by twilwa (Modal runtime portions excluded).
3a64348772