ci: don't let ruff fix unused imports (rule F401) (#9839)

This prevents `ruff --fix` from fixing unused imports by deleting them.

This avoids scenarios where `hatch run lint:fmt` can lead to broken
tests (eg: tests that trigger side effects with `import ddtrace`) if
someone's workflow is "run tests, then format" instead of "format, then
run tests".

This does not introduce possible regressions because unused imports that
aren't marked with `# noqa:F401`, for example) will still fail `ruff`
checks:
```
tests/internal/test_module.py:512:12: F401 `ddtrace` imported but unused
```

The only difference is that they will not automatically be deleted by
[the call to `ruff --fix` in `hatch run
lint:fmt`](https://github.com/DataDog/dd-trace-py/blob/a09067333cf573a0636d0107e13363d4048fbd67/hatch.toml#L47).

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

pyproject.toml

@@ -268,3 +268,7 @@ relative-imports-order = "furthest-to-closest"
 # Exclude typing stubs as vertical line spacing incompatibility with black
 # See: https://github.com/astral-sh/ruff/pull/6501
 "*.pyi" = ["I001"]
+
+[tool.ruff.lint]
+# Do not auto-fix unused imports (as this deletes things like import ddtrace)
+unfixable = ["F401"]