feat(gateway): add allowed_{chats,channels,rooms} whitelist to Telegram, Mattermost, Matrix, DingTalk

Mirrors the Slack `allowed_channels` feature (PR #7401) and Discord's
`allowed_channels` (PR #7044) across the remaining group-capable platforms.
All five platforms (Slack + Discord + the four added here) now follow the
same pattern: primary config via config.yaml, env-var fallback as an escape
hatch — matching the project policy that .env is for secrets only and
behavioral settings belong in config.yaml.

Also fixes a duplicate `slack` key in DEFAULT_CONFIG introduced by PR
#7401 (the later entry silently overwrote `allowed_channels`, `require_mention`,
and `free_response_channels` at dict-literal evaluation time).

Platforms added:
- Telegram: `telegram.allowed_chats` (env alias: `TELEGRAM_ALLOWED_CHATS`)
- Mattermost: `mattermost.allowed_channels` (env alias: `MATTERMOST_ALLOWED_CHANNELS`)
- Matrix: `matrix.allowed_rooms` (env alias: `MATRIX_ALLOWED_ROOMS`)
- DingTalk: `dingtalk.allowed_chats` (env alias: `DINGTALK_ALLOWED_CHATS`)

Mattermost and Matrix previously had NO config.yaml bridging for any of
their gating settings; this PR adds `load_gateway_config` bridges for them
(Mattermost gets require_mention + free_response_channels + allowed_channels;
Matrix gets allowed_rooms on top of its existing bridges for require_mention
and free_response_rooms).

Semantics identical everywhere:
- Empty = no restriction (fully backward compatible).
- Non-empty = hard whitelist: non-listed chats are silently ignored,
  even when the bot is @mentioned.
- DMs bypass the check entirely.

DEFAULT_CONFIG merges the duplicate `slack` block and adds new `mattermost`
and `matrix` blocks so all gating settings surface in defaults.

Not included: Feishu (has its own per-chat `chat_rules` system that covers
this use case differently), WhatsApp (already has `group_allow_from` via
`group_policy: allowlist`), pure-DM platforms (Signal, SMS, BlueBubbles,
Yuanbao — no group concept).

gateway/platforms/telegram.py

@@ -2771,6 +2771,20 @@ class TelegramAdapter(BasePlatformAdapter):
             return {str(part).strip() for part in raw if str(part).strip()}
         return {part.strip() for part in str(raw).split(",") if part.strip()}
 
+    def _telegram_allowed_chats(self) -> set[str]:
+        """Return the whitelist of group/supergroup chat IDs the bot will respond in.
+
+        When non-empty, group messages from chats NOT in this set are silently
+        ignored — even if the bot is @mentioned.  DMs are never filtered.
+        Empty set means no restriction (fully backward compatible).
+        """
+        raw = self.config.extra.get("allowed_chats")
+        if raw is None:
+            raw = os.getenv("TELEGRAM_ALLOWED_CHATS", "")
+        if isinstance(raw, list):
+            return {str(part).strip() for part in raw if str(part).strip()}
+        return {part.strip() for part in str(raw).split(",") if part.strip()}
+
     def _telegram_ignored_threads(self) -> set[int]:
         raw = self.config.extra.get("ignored_threads")
         if raw is None:
@@ -2919,13 +2933,16 @@ class TelegramAdapter(BasePlatformAdapter):
         """Apply Telegram group trigger rules.
 
         DMs remain unrestricted. Group/supergroup messages are accepted when:
+        - the chat passes the ``allowed_chats`` whitelist (when set)
         - the chat is explicitly allowlisted in ``free_response_chats``
         - ``require_mention`` is disabled
         - the message replies to the bot
         - the bot is @mentioned
         - the text/caption matches a configured regex wake-word pattern
 
-        When ``require_mention`` is enabled, slash commands are not given
+        When ``allowed_chats`` is non-empty, it acts as a hard gate — messages
+        from any chat not in the list are ignored regardless of the other
+        rules.  When ``require_mention`` is enabled, slash commands are not given
         special treatment — they must pass the same mention/reply checks
         as any other group message.  Users can still trigger commands via
         the Telegram bot menu (``/command@botname``) or by explicitly
@@ -2934,6 +2951,14 @@ class TelegramAdapter(BasePlatformAdapter):
         """
         if not self._is_group_chat(message):
             return True
+        # allowed_chats check (whitelist — must pass before other gating).
+        # When set, group messages from chats NOT in this whitelist are
+        # silently ignored, even if @mentioned.  DMs are already excluded above.
+        allowed = self._telegram_allowed_chats()
+        if allowed:
+            chat_id_str = str(getattr(getattr(message, "chat", None), "id", ""))
+            if chat_id_str not in allowed:
+                return False
         thread_id = getattr(message, "message_thread_id", None)
         if thread_id is not None:
             try: