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.

pyproject.toml

@@ -38,6 +38,7 @@ dev = [
     "ruff>=0.1",
     "mypy>=1.0",
     "types-PyYAML>=6.0",
+    "pandas-stubs>=2.0",
 ]
 
 [project.scripts]