`gateway/run.py::_UPDATE_ALLOWED_PLATFORMS` was a hardcoded frozenset
listing every messaging platform allowed to invoke the `/update` slash
command. Plugin-migrated platforms (currently Discord and Mattermost,
soon also Home Assistant via #32500) declare `allow_update_command=True`
on their `PlatformEntry`, and `_handle_update_command` already falls
back to the registry when a platform isn't in the frozenset. The result
was a silent redundancy: those entries said "allowed" twice, and the
registry flag was a no-op for them in practice.
- Removed `Platform.DISCORD` and `Platform.MATTERMOST` from the frozenset.
- Updated the docstring to make the split explicit (built-ins live in
the frozenset; plugins use `allow_update_command` on the registry entry).
The remaining frozenset entries are all still built-in platforms living
under `gateway/platforms/` today. Future plugin migrations should drop
their entry from the frozenset as part of the migration PR (or in a
sibling chore PR like this one).
Added a `TestUpdateCommandPlatformGate` test class that pins down all
three branches of the gate so future changes don't silently regress:
- Programmatic interfaces (`Platform.WEBHOOK`, `Platform.API_SERVER`)
must remain blocked.
- Plugin-migrated platforms (Discord, Mattermost) must pass via the
registry fallback.
- Built-in platforms in the hardcoded frozenset (Telegram) must
still pass without needing the registry.
The gate previously had zero direct test coverage — its only existing
coverage was `test_no_adapter_for_platform` which exercised a different
code path.
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