fix(whatsapp): reuse persistent aiohttp session across requests (#3818)

Replace per-request aiohttp.ClientSession() in every WhatsApp adapter
method with a single persistent self._http_session, matching the pattern
used by Mattermost, HomeAssistant, and SMS adapters.

Changes:
- Create self._http_session in connect(), close in disconnect()
- All bridge HTTP calls (send, edit, send-media, typing, get_chat_info,
  poll_messages) now use the shared session
- Explicitly cancel _poll_task on disconnect() instead of relying
  solely on self._running = False
- Health-check sessions in connect() remain ephemeral (persistent
  session not yet created at that point)
- Remove per-method ImportError guards for aiohttp (always available
  when gateway runs via [messaging] extras)

Salvaged from PR #1851 by Himess. The _poll_task storage was already
on main from PR #3267; this adds the disconnect cancellation and the
persistent session.

Tests: 4 new tests for session close, already-closed skip, poll task
cancellation, and done-task skip.

tests/gateway/test_whatsapp_connect.py

@@ -63,6 +63,7 @@ def _make_adapter():
     adapter._background_tasks = set()
     adapter._auto_tts_disabled_chats = set()
     adapter._message_queue = asyncio.Queue()
+    adapter._http_session = None
     return adapter
 
 
@@ -219,6 +220,7 @@ class TestBridgeRuntimeFailure:
         fatal_handler = AsyncMock()
         adapter.set_fatal_error_handler(fatal_handler)
         adapter._running = True
+        adapter._http_session = MagicMock()  # Persistent session active
         mock_fh = MagicMock()
         adapter._bridge_log_fh = mock_fh
 
@@ -242,6 +244,7 @@ class TestBridgeRuntimeFailure:
         fatal_handler = AsyncMock()
         adapter.set_fatal_error_handler(fatal_handler)
         adapter._running = True
+        adapter._http_session = MagicMock()  # Persistent session active
         mock_fh = MagicMock()
         adapter._bridge_log_fh = mock_fh
 
@@ -417,3 +420,83 @@ class TestKillPortProcess:
         with patch("gateway.platforms.whatsapp._IS_WINDOWS", True), \
              patch("gateway.platforms.whatsapp.subprocess.run", side_effect=OSError("no netstat")):
             _kill_port_process(3000)  # must not raise
+
+
+# ---------------------------------------------------------------------------
+# Persistent HTTP session lifecycle
+# ---------------------------------------------------------------------------
+
+class TestHttpSessionLifecycle:
+    """Verify persistent aiohttp.ClientSession is created and cleaned up."""
+
+    @pytest.mark.asyncio
+    async def test_session_closed_on_disconnect(self):
+        """disconnect() should close self._http_session."""
+        adapter = _make_adapter()
+        mock_session = AsyncMock()
+        mock_session.closed = False
+        adapter._http_session = mock_session
+        adapter._poll_task = None
+        adapter._bridge_process = None
+        adapter._running = True
+        adapter._session_lock_identity = None
+
+        await adapter.disconnect()
+
+        mock_session.close.assert_called_once()
+        assert adapter._http_session is None
+
+    @pytest.mark.asyncio
+    async def test_session_not_closed_when_already_closed(self):
+        """disconnect() should skip close() when session is already closed."""
+        adapter = _make_adapter()
+        mock_session = AsyncMock()
+        mock_session.closed = True
+        adapter._http_session = mock_session
+        adapter._poll_task = None
+        adapter._bridge_process = None
+        adapter._running = True
+        adapter._session_lock_identity = None
+
+        await adapter.disconnect()
+
+        mock_session.close.assert_not_called()
+        assert adapter._http_session is None
+
+    @pytest.mark.asyncio
+    async def test_poll_task_cancelled_on_disconnect(self):
+        """disconnect() should cancel the poll task."""
+        adapter = _make_adapter()
+        mock_task = MagicMock()
+        mock_task.done.return_value = False
+        mock_task.cancel = MagicMock()
+        mock_future = asyncio.Future()
+        mock_future.set_exception(asyncio.CancelledError())
+        mock_task.__await__ = mock_future.__await__
+        adapter._poll_task = mock_task
+        adapter._http_session = None
+        adapter._bridge_process = None
+        adapter._running = True
+        adapter._session_lock_identity = None
+
+        await adapter.disconnect()
+
+        mock_task.cancel.assert_called_once()
+        assert adapter._poll_task is None
+
+    @pytest.mark.asyncio
+    async def test_disconnect_skips_done_poll_task(self):
+        """disconnect() should not cancel an already-done poll task."""
+        adapter = _make_adapter()
+        mock_task = MagicMock()
+        mock_task.done.return_value = True
+        adapter._poll_task = mock_task
+        adapter._http_session = None
+        adapter._bridge_process = None
+        adapter._running = True
+        adapter._session_lock_identity = None
+
+        await adapter.disconnect()
+
+        mock_task.cancel.assert_not_called()
+        assert adapter._poll_task is None