fix: add parallel UT coverage reporting and recover Codecov

[JasonW404]

Summary

• Add a parallel unit-test runner with file-level subprocess isolation and configurable worker count/timeouts.
• Keep detailed pass-rate accounting for per-file test execution.
• Generate combined branch coverage reports through the Coverage API, including terminal, HTML, and test/coverage.xml outputs.
• Keep deployment script checks and deployment path triggers in the unit-test workflow.
• Recover the Codecov upload path that has been failing since April by upgrading the Codecov action, selecting the linux-arm64 uploader explicitly, failing CI on upload errors, and pinning the action to a full commit SHA for SonarCloud compliance.

Codecov Recovery

Historical CI logs showed the coverage XML was generated successfully, but codecov/codecov-action@v4 failed on the ARM runner after selecting an invalid uploader path. This PR updates the uploader configuration so the UT job uploads test/coverage.xml successfully and restores the codecov/patch PR check.

Validation

• Parsed .github/workflows/auto-unit-test.yml with PyYAML.
• Ran backend/.venv/bin/python -m py_compile test/run_all_test.py.
• Smoke-ran the UT runner with NEXENT_PYTEST_TARGETS=test/backend/consts/test_error_code.py, producing 79 passed and test/coverage.xml.
• Confirmed PR CI showed test passing and codecov/patch passing after the Codecov workflow update.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[Copilot]

Pull request overview

This PR updates the unit-test/coverage pipeline to better support parallelized pytest execution while ensuring combined coverage artifacts are generated consistently and successfully uploaded to Codecov on the ARM runner.

Changes:

• Adjust coverage generation so the combined .coverage data is created first and XML is generated via the Coverage Python API during the reporting step.
• Add a codecov.yml configuration to enforce project/patch coverage status behavior and PR commenting layout.
• Update the unit-test GitHub Actions workflow to pin/upgrade the Codecov action, explicitly select the ARM64 uploader OS, and fail CI on upload errors to restore the codecov/patch check.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/run_all_test.py	Moves XML generation into the Coverage API reporting step after combining parallel coverage data.
codecov.yml	Adds Codecov status/comment configuration for project and patch coverage reporting.
.github/workflows/auto-unit-test.yml	Pins/upgrades Codecov upload action, adds uploader OS override input, and enforces failing on upload errors.

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