fix(ci): set COVERAGE_FILE so unit coverage uploads correctly

[LalatenduMohanty]

The switch from coverage run -m pytest to pytest --cov (#1219)
caused pytest-cov to write a single .coverage file. The CI upload
step uses the glob .coverage.*, which requires a suffix after the
dot — so the unit test coverage data was never uploaded. Only e2e
coverage data (which still produces .coverage.* files via subprocess
instrumentation) reached the combine step, dropping overall coverage
from ~93% to 37% and failing the --fail-under=60 gate.

Fix by setting COVERAGE_FILE with a per-job suffix so pytest-cov
writes directly to e.g. .coverage.unit.py3.12.ubuntu-latest, which
the upload glob matches.

Also remove tests/ from [tool.coverage.run].source to measure
only production code coverage.

Closes: #1222

• PR follows CONTRIBUTING.md guidelines

[coderabbitai]

📝 Walkthrough

Walkthrough

The CI workflow sets COVERAGE_FILE to a matrix-specific filename (including Python version and OS) in the unit test job, ensuring each matrix combination writes a uniquely named coverage file that matches the .coverage.* upload glob. The pyproject.toml coverage path configuration replaces the tests/ path mapping with a .hatch/**/site-packages/fromager mapping to correctly resolve paths from hatch-installed package environments.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the CI coverage-file fix and matches the main change.
Linked Issues check	✅ Passed	The changes address the broken coverage gate by generating uploadable per-job coverage files and updating coverage path handling.
Out of Scope Changes check	✅ Passed	The diff stays within CI coverage handling and coverage config, with no unrelated changes.
Description check	✅ Passed	The description clearly matches the coverage-file and coverage-source changes in the pull request.

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