feat: add Python test coverage

[fernandolins]

Linked issue

Closes #4838

Summary/motivation

Adds coverage.py-based test coverage for both Python test suites (pylib and qt). Introduces just test-py --coverage and just test-py --coverage --html, plus the just test --coverage.

Coverage reports are written to out/coverage/.

How to test

# Existing behavior unchanged
just test-py

# Terminal summary + enforces thresholds
just test-py --coverage

# Terminal summary + HTML reports under out/coverage/
just test-py --coverage --html

# Umbrella (Python only for now)
just test --coverage
just test --coverage --html

Checklist (minimum)

• I ran ./ninja check or an equivalent relevant check locally.

Details

• coverage dependency pinned to >=7.13.5 in pyproject.toml.
• The coverage umbrella recipe currently delegates to Python only for now

Before / after behavior

Before: no just test-py, no coverage support.
After: just test-py runs Python tests via ninja; just test-py --coverage
runs them with coverage.py and enforces minimum line coverage.

[abdnh]

I added Windows support. Can you check the changes?

Are we planning to enforce 100% coverage? Do we have a plan on how to tackle Qt?

[fernandolins]

Thanks @abdnh!

Ideally, we wish to achieve 100% coverage but not in a row.

For the first moment, we want to add a coverage test to ensure which parts of the codebase are tested and which are not. Then, with this information, we'll plan the next steps.