feat(gateway): add Telegram notification mode to suppress intermediate push notifications

Add a configurable notifications mode for the Telegram platform adapter
that controls which messages trigger push notifications.

- display.platforms.telegram.notifications: "all" (default) | "important"
- HERMES_TELEGRAM_NOTIFICATIONS env var override
- In "important" mode, all sends use disable_notification=True except:
  - Approvals (send_exec_approval) and slash confirmations
  - Final response messages (metadata["notify"]=True)
- Zero overhead in default "all" mode
- Zero impact on non-Telegram platforms

Closes #22771

tests/gateway/test_base_topic_sessions.py

@@ -130,8 +130,8 @@ class TestBasePlatformTopicSessions:
             {
                 "chat_id": "-1001",
                 "content": "ack",
-                "reply_to": "1",
-                "metadata": {"thread_id": "17585"},
+                "reply_to": None,
+                "metadata": {"thread_id": "17585", "notify": True},
             }
         ]
         assert typing_calls == [