Add parallel live simulation coverage

[anth-volk]

Fixes #2400

Summary

• keep candidate health, metadata, and the stateful v2 policy lifecycle checks in the serial staging integration job
• run the non-stateful live test selection inside one GitHub Actions job using two pytest-xdist worker processes with dynamic per-test scheduling
• limit concurrency explicitly with -n 2, use --dist load, and schedule one test at a time with --maxschedchunk=1
• use the exact tagged candidate URL and collision-resistant probe identifiers containing the deployment tag, workflow run, run attempt, and a generated worker-session UUID
• add fixed 2025 household assertions for federal EITC, federal CTC, CalEITC, and Universal Credit for a household in Scotland
• add society-wide California CalEITC and Scotland Universal Credit reforms while retaining the existing Utah and budget-window coverage
• remove the obsolete Enhanced CPS configuration requirement from the linked issue; society-wide requests use the currently certified default dataset
• require both the serial staging checks and the pytest-xdist live-test job to pass before staging promotion

Validation

• make format
• Ruff checks on all changed Python files
• pytest tests/unit/test_cloud_run_deploy_scripts.py -q (102 passed)
• executed the live-test runner locally with pytest-xdist: 2 workers distributed all 11 tests; all skipped because no deployed API URL was supplied
• validated .github/workflows/push.yml with actionlint
• validated the live-test runner with bash -n
• parsed all new JSON fixtures with jq
• resolved both reform parameter paths against the pinned US and UK packages
• calculated the expected household results locally with the pinned US and UK packages
• ran the changelog-fragment and diff checks

The deployed live checks were not run locally because they require a newly deployed tagged staging candidate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.18%. Comparing base (37e168a) to head (4b44816).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3820       +/-   ##
===========================================
+ Coverage   46.26%   87.18%   +40.92%     
===========================================
  Files         135      150       +15     
  Lines        7963     8935      +972     
  Branches     1326     1542      +216     
===========================================
+ Hits         3684     7790     +4106     
+ Misses       3990      703     -3287     
- Partials      289      442      +153     

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[anth-volk]

Deployment validation for this PR currently depends on resolving the pre-existing failure in tests/integration/test_live_phase10_staging.py::test_live_phase10_activation_failure_and_retry on master (failed workflow run). Anthony is working on that failure; this PR should remain blocked until the existing deployment sequence completes successfully.