fix: add null guards for _client in Python v3 handlers

The ty type checker correctly identifies that self._client can be None.
Add explicit guards before the .request() calls in the v3 broadcast
event handlers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: patniko

python/copilot/client.py

@@ -1538,6 +1538,8 @@ async def _handle_external_tool_requested(self, session_id: str, event_dict: dic
                     session_id, tool_call_id, tool_name, arguments, handler
                 )
 
+            if not self._client:
+                return
             await self._client.request(
                 "session.tools.handlePendingToolCall",
                 {
@@ -1583,6 +1585,8 @@ async def _handle_permission_requested_event(self, session_id: str, event_dict:
                 return
 
             result = await session._handle_permission_request(permission_request)
+            if not self._client:
+                return
             await self._client.request(
                 "session.permissions.handlePendingPermissionRequest",
                 {