[Channels] E2E verification and test coverage for Telegram + Discord

[Al629176]

Parent: #285

Add E2E verification that Channels page + Telegram/Discord integrations work end‑to‑end.

Tasks

• Define a manual E2E test script (Telegram + Discord) and add it to docs.
• Add automated tests where feasible (e.g. mocked adapters or test bots).
• Verify:
  • Channel selection affects outbound route.
  • Telegram send/receive (including Managed DM).
  • Discord send/receive.

Acceptance criteria

• Documented, repeatable E2E checklist exists.
• CI and/or manual run can confirm channels integration is working before releases.