chore(gateway): drop HOMEASSISTANT from /update allowlist (#40736)

Home Assistant is a bundled plugin now (#40709) and declares
allow_update_command=True on its PlatformEntry. The registry fallback
in _handle_update_command already covers it, so the frozenset entry is
a redundant double-allow — same cleanup #40711 did for Discord and
Mattermost. Adds a registry-fallback test mirroring the existing
discord/mattermost cases.

gateway/run.py

@@ -14976,7 +14976,7 @@ class GatewayRunner:
     _UPDATE_ALLOWED_PLATFORMS = frozenset({
         Platform.TELEGRAM, Platform.SLACK, Platform.WHATSAPP,
         Platform.SIGNAL, Platform.MATRIX,
-        Platform.HOMEASSISTANT, Platform.EMAIL, Platform.SMS, Platform.DINGTALK,
+        Platform.EMAIL, Platform.SMS, Platform.DINGTALK,
         Platform.FEISHU, Platform.WECOM, Platform.WECOM_CALLBACK, Platform.WEIXIN, Platform.BLUEBUBBLES, Platform.QQBOT, Platform.LOCAL,
     })