feat: auto-reconnect failed gateway platforms with exponential backoff (#2584)

When a messaging platform fails to connect at startup (e.g. transient DNS
failure) or disconnects at runtime with a retryable error, the gateway now
queues it for background reconnection instead of giving up permanently.

- New _platform_reconnect_watcher background task runs alongside the
  existing session expiry watcher
- Exponential backoff: 30s, 60s, 120s, 240s, 300s cap
- Max 20 retry attempts before giving up on a platform
- Non-retryable errors (bad auth token, etc.) are not retried
- Runtime disconnections via _handle_adapter_fatal_error now queue
  retryable failures instead of triggering gateway shutdown
- On successful reconnect, adapter is wired up and channel directory
  is rebuilt automatically

Fixes the case where a DNS blip during gateway startup caused Telegram
and Discord to be permanently unavailable until manual restart.

tests/gateway/test_runner_fatal_adapter.py

@@ -66,7 +66,9 @@ async def test_runner_requests_clean_exit_for_nonretryable_startup_conflict(monk
 
 
 @pytest.mark.asyncio
-async def test_runner_requests_failure_exit_for_retryable_runtime_fatal(monkeypatch, tmp_path):
+async def test_runner_queues_retryable_runtime_fatal_for_reconnection(monkeypatch, tmp_path):
+    """Retryable runtime fatal errors queue the platform for reconnection
+    instead of shutting down the gateway."""
     config = GatewayConfig(
         platforms={
             Platform.WHATSAPP: PlatformConfig(enabled=True, token="token")
@@ -87,7 +89,7 @@ async def test_runner_requests_failure_exit_for_retryable_runtime_fatal(monkeypa
 
     await runner._handle_adapter_fatal_error(adapter)
 
-    assert runner.should_exit_cleanly is False
-    assert runner.should_exit_with_failure is True
-    assert "exited unexpectedly" in runner.exit_reason
-    runner.stop.assert_awaited_once()
+    # Should NOT shut down — platform is queued for reconnection
+    runner.stop.assert_not_awaited()
+    assert Platform.WHATSAPP in runner._failed_platforms
+    assert runner._failed_platforms[Platform.WHATSAPP]["attempts"] == 0