Don't return code actions for non-Python documents (#23905)
## Summary
- Add early return in the code action handler for non-Python documents
(e.g., markdown), matching the existing guard pattern in `lint::check()`
and `fix::fix_all()`
- The server was incorrectly returning `source.fixAll.ruff` and
`source.organizeImports.ruff` actions for files like `.md`
Closes https://github.com/astral-sh/ruff/issues/23861
## Test plan
- Added unit tests in `code_action.rs`:
- `no_code_actions_for_markdown` — verifies empty response for `.md`
files
- `code_actions_for_python` — verifies non-empty response for `.py`
files
```sh
cargo test -p ruff_server -- code_action::tests
```
