Warn about writing to a closed stream

Author: jhrozek

src/codegate/providers/copilot/provider.py

@@ -666,6 +666,9 @@ def _process_chunk(self, chunk: bytes):
             self.stream_queue.put_nowait(record)
 
     def _proxy_transport_write(self, data: bytes):
+        if not self.proxy.transport or self.proxy.transport.is_closing():
+            logger.error("Proxy transport not available")
+            return
         self.proxy.transport.write(data)
 
     def data_received(self, data: bytes) -> None: