Refactor: Add explicit return type to run_server function

[pyob-bot]

Summary of Changes

• Updated the run_server nested function in src/pyob/entrance_mixins.py to include an explicit return type annotation (-> None).
• No functional behavior changes; this enhances type clarity and static analysis support.

Technical Impact

• Improves code readability and maintainability by making the intended return value clear.
• Enables better integration with type checking tools (e.g., mypy, Pyright) and IDE autocompletion.
• No runtime impact or changes to existing functionality; the server behavior remains identical.