ci(build): consolidate pytest workflow steps

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Summary

• Consolidate repeated pytest, JUnit, Trunk upload, and failure handling into a local composite action.
• Share the Docker build cache scope between integration prebuilds and publish builds.
• Extend CI path and pinned-action coverage to local workflow actions.

What changed

• Added .github/actions/run-pytest for real pytest suite execution with optional Trunk upload.
• Updated unit and integration CI jobs to call the local action instead of duplicating shell blocks.
• Added pytest coverage for workflow consolidation, shared cache scope, and local-action change detection.

Why

• Reduces duplicated CI logic and drift.
• Improves Docker cache reuse without removing validation coverage.
• Keeps checks as real pytest suite runs, not smoke tests.

Validation

• .venv-local/bin/python -m pytest tests/unit tests/template -q
• trunk fmt .github/workflows/build.yml .github/actions/run-pytest/action.yml tests/unit/test_workflow_ci_efficiency.py
• trunk check --ci .github/workflows/build.yml .github/actions/run-pytest/action.yml tests/unit/test_workflow_ci_efficiency.py