Backport !247590 to 19.1: Fix pipeline test report timeout on large pipelines

What does this MR do and why?

Backport of !247590 (merged) ('Fix pipeline test report timeout on large pipelines') to the 19-1-stable-ee branch.

Ci::Pipeline#accessible_test_reports / #accessible_test_report_summary loaded every test-report build (self and same-project descendants) at once and evaluated the read_job_artifacts policy per artifact, causing Rack::Timeout on large pipelines. The fix streams builds with find_each and memoizes the per-artifact authorization by [project, accessibility, file_type]; the summary path collects only build ids. Which reports a user may read is unchanged.

Cherry-picked squash commit f4bd818b6bbbfa164814ee8ad6b6da119e48d3d3 from master onto 19-1-stable-ee. Applied cleanly with no conflicts.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
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• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.