The Phase 1 exit gate requires BOTH Discord and Telegram to round-trip
through the relay stub, but test_relay_roundtrip.py only covered Discord.
Add the Telegram companion exercising its distinct discriminator profile:
- no guild_id — two chats isolate on chat_id alone
- forum topics share one chat_id and isolate by thread_id (the Telegram
analog of Discord per-guild isolation), shared across participants by
default (thread_sessions_per_user=False)
- DM isolation by chat_id
- utf16 len_unit + markdown_v2 dialect round-trip and configure the adapter
- outbound send round-trips through the stub
Proves the CapabilityDescriptor + build_session_key generalize beyond
Discord, not just the struct (which the descriptor unit tests already
covered).
