fix(gateway): derive channel directory platforms from enum instead of hardcoded list

Six platforms (matrix, mattermost, dingtalk, feishu, wecom, homeassistant)
were missing from the session-based discovery loop, causing /channels and
send_message to return empty results on those platforms.

Instead of adding them to the hardcoded tuple (which would break again when
new platforms are added), derive the list dynamically from the Platform enum.
Only infrastructure entries (local, api_server, webhook) are excluded;
Discord and Slack are skipped automatically because their direct builders
already populate the platforms dict.

Reported by sprmn24 in PR #7416.

gateway/channel_directory.py

@@ -76,10 +76,15 @@ def build_channel_directory(adapters: Dict[Any, Any]) -> Dict[str, Any]:
         except Exception as e:
             logger.warning("Channel directory: failed to build %s: %s", platform.value, e)
 
-    # Telegram, WhatsApp & Signal can't enumerate chats -- pull from session history
-    for plat_name in ("telegram", "whatsapp", "signal", "weixin", "email", "sms", "bluebubbles"):
-        if plat_name not in platforms:
-            platforms[plat_name] = _build_from_sessions(plat_name)
+    # Platforms that don't support direct channel enumeration get session-based
+    # discovery automatically.  Skip infrastructure entries that aren't messaging
+    # platforms — everything else falls through to _build_from_sessions().
+    _SKIP_SESSION_DISCOVERY = frozenset({"local", "api_server", "webhook"})
+    for plat in Platform:
+        plat_name = plat.value
+        if plat_name in _SKIP_SESSION_DISCOVERY or plat_name in platforms:
+            continue
+        platforms[plat_name] = _build_from_sessions(plat_name)
 
     directory = {
         "updated_at": datetime.now().isoformat(),