Add Commons and Connections unit coverage

[bernalde]

Summary

• Add fast @pytest.mark.unit test files for PharmaPy.Commons and PharmaPy.Connections helper behavior.
• Cover state unpacking, discretized finite-volume state reconstruction, profile flattening, FVM flux helpers, flowsheet parsing, input splitting, missing-field defaults, interpolation, and remaining-state filling with hand-checkable NumPy fixtures.
• Ignore local .claude/ and .codex/ agent directories so they do not dirty the working tree.

Refs #7.

Coverage Scope

This is one small Wave 2 test-only slice for issue #7. It does not close the full coverage roadmap or set the repository-wide coverage target; it adds fifteen fast Commons/Connections unit tests that run in the existing Assimulo-free lane.

Tests

• pytest tests/test_commons_helpers.py -q
• pytest tests/test_connections_helpers.py -q
• pytest tests/ -m "not assimulo" -q -rs
• pytest --collect-only -q -rs
• pytest tests/ -m unit -q -rs
• pytest -q -rs

Branch Hygiene

• Base branch: master
• Source branch: test/issue-7-unit-coverage
• Branch point: origin/master at bff897f
• Upstream status: upstream/master at 400e1e1 is included in the fork base.
• Stacked status: unstacked; no prerequisite PRs.