Investigate intermittent regression test failures in GitHub Actions CI

[harryswift01]

Summary

This PR updates the pull request CI workflow to help isolate intermittent CI-only regression failures by removing potentially stale cached regression data, clearing test caches, and standardizing numerical threading settings.

Changes

CI environment stabilization:

• Set deterministic numerical threading environment variables across the workflow.
• Set PYTHONHASHSEED=0 for stable Python hashing behavior.

Regression cache isolation:

• Temporarily disabled .testdata caching for fast regression tests.
• Clear .pytest_cache, .testdata, and __pycache__ before regression execution.
• Run pytest with --cache-clear.

Regression execution debugging:

• Temporarily run fast regression tests with -n 1 to rule out pytest-xdist parallel interference.
• Preserve failure artifact upload for debugging failed regression runs.

Impact

• Helps determine whether CI flakes are caused by stale cached test data, pytest cache state, parallel test interference, or numerical backend differences.
• Does not change application logic or regression expected values.
• Provides a smaller, safer debugging step before making numerical algorithm changes.