Report test-code coverage in CI

[anshurajbisoyi98-ctrl]

What problem do you want to solve?

Related to #564 and the coverage-reporting step requested in #571.

The CI coverage task currently measures only src, and Codecov explicitly ignores tests. This change adds --cov=tests alongside --cov=src and removes the test-directory exclusion from Codecov, making uncovered test helpers visible for review.

This is the reporting step only. It does not remove test helpers, add coverage exclusions, enforce 100% test coverage, or resolve all of #564. Categorizing uncovered test code can follow once the full CI report is available.

Validation on macOS with Python 3.13:

• 28 focused tests passed (tests/optimagic/test_decorators.py and tests/optimagic/test_batch_evaluators.py).
• Confirmed that the generated XML report contains source and test files.
• All applicable pre-commit hooks passed for the two changed files.
• git diff --check passed.

The full test suite and remote CI have not yet been verified.

Todo

• Target the right branch and pick an appropriate title.
• Close the relevant issue automatically: not applicable because this PR addresses only the reporting step of Ensure 100% coverage in tests #564.
• Verify the full CI run and uploaded Codecov report after submission.

AI assistance

Codex assisted with implementation and local verification, .