Follow-ups on top of #26016's expensive-model guard:
- gateway/slash_commands.py: typed '/model <name>' now routes through the
expensive-model confirmation gate (slash-confirm buttons / text fallback)
instead of bypassing the guard the pickers enforce. Cancel leaves the
session override and --global config untouched.
- telegram/discord/web_server: run expensive_model_warning() via
asyncio.to_thread — it can hit models.dev or a /models endpoint on a
cache miss, which would otherwise block the event loop.
- telegram: picker callback no longer toasts 'Model switched!' when the
switch callback raised (both mm: and mc: paths).
- tests: new tests/gateway/test_model_command_expensive_confirm.py pins
the typed-path gate (prompt, confirm-once, cancel, cheap-model no-op).
