Draft: Update PixelMaskMetric with masked-pixel diagnostics

[jeipollack]

Summary

This PR extends the PixelMaskMetric class to compute four diagnostics describing pixel-mask coverage across the image and within a configurable aperture around the source:

• Total masked pixels — number of masked pixels in the image.
• Total masked fraction — fraction of image pixels that are masked.
• Aperture masked pixels — number of masked pixels within the configurable aperture
• Aperture masked fraction — fraction of aperture pixels that are masked.

Closes #254

What’s changed

• Implements the four pixel-mask diagnostics described above.
• Adds unit and integration tests in qc_metrics_pixel_masks_test.py.
• Adds a pipeline integration test in pipeline_test.py that runs the pixel-mask metric.
• Refactors the test module by migrating shared fixtures to conftest.py and moving the PSFDataset helper to a new test_utils.py module.

How to test / verify

• Check CI passes.

• Review the implementation and test coverage, with particular attention to:

  • the definition of the aperture and its centre conventions;
  • the distinction between total and aperture-restricted masked-pixel diagnostics;
  • the handling of invalid aperture configurations;
  • integration with the dictionary-based QualityMetric.compute() API and QualityControlResult.

Scope

Indicate the type of PR:

• Feature
• Bug fix
• Hotfix
• Documentation / process change
• Internal / refactor
• Release

Optionally, note if this PR is part of a larger milestone or set of related PRs.

Changelog

• Changelog fragment added (if applicable)

Reviewer Checklist

Reviewers should confirm the following before approving and merging:

• The PR targets the correct base branch (develop, or main for release PRs)
• The PR is assigned to the developer
• Appropriate labels are applied
• The PR is included in relevant projects and/or milestones
• Description clearly explains what has changed
• Issue references included, if applicable
• Code and documentation adhere to current standards (ruff)
• Documentation updates included, if relevant
• CI tests are passing
• All reviewer comments have been addressed

Next Steps / Notes (if applicable)

Next, the rejection policy will be implemented and tested for this quality metric.