[1/4] Add test runner controls and timing reports

[jlnav]

• Adds coverage toggles, fail-fast, test filters, list mode, sharding, and slow-test timing reports.
• Validates standalone TESTSUITE_* metadata and adds focused runner unit tests.
• Replaces the duplicated deduplication proposal with a measured test-acceleration strategy.
• Stack: parent fix mfkg test issue. clarify asking questions in AGENTS.md #1780; child [2/4] Split pull request CI by responsibility #1783.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.29%. Comparing base (c090e16) to head (0b9b296).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1784      +/-   ##
===========================================
- Coverage    89.31%   89.29%   -0.03%     
===========================================
  Files           83       83              
  Lines         8927     8927              
  Branches      1384     1384              
===========================================
- Hits          7973     7971       -2     
- Misses         718      719       +1     
- Partials       236      237       +1     

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[jlnav]

Follow-up fix pushed in commit 096c5761d: added mypy >=1.19.1,<2 to the shared Pixi basic feature and regenerated pixi.lock. Verified pixi run -e py312 mypy succeeds with no issues. The combined local Flake8 command still reports an unrelated warning in the pre-existing untracked libensemble/tests/unit_tests/test_features.py file (missing final newline); that file was left untouched.