Fix TCP server response handling and add pandas-stubs

- Always send a response (even empty) to prevent client timeouts
- Add pandas-stubs to dev dependencies for mypy type checking
- Server now sends newline-terminated response for all commands

This fixes the mypy CI failure. Integration test failures still need
investigation - likely due to event loop blocking when mixing sync/async.

src/py_dvt_ate/instruments/transport/server.py

@@ -217,10 +217,13 @@ class InstrumentServer:
                 response = instrument.process(command)
 
                 # Send response with newline terminator
+                # Always send a response, even if empty (for acknowledgment)
+                writer.write(f"{response}\n".encode())
+                await writer.drain()
                 if response:
-                    writer.write(f"{response}\n".encode())
-                    await writer.drain()
                     logger.debug("Port %d sent: %s", port, response)
+                else:
+                    logger.debug("Port %d sent: <empty response>", port)
 
         except asyncio.CancelledError:
             logger.debug("Client handler cancelled for port %d", port)