Fix: handle empty/zero-line classes in Cobertura parser + add regression tests

[afinetooth]

What this does

• Fixes a crash in the Cobertura parser (Arithmetic overflow) when a file has:

  • No entries at all, or
  • Only line="0" entries (example: <package>/__init__.py inside customer’s coverage XML).

• Adds two tiny fixtures (cobertura-empty-lines.xml, cobertura-zero-only.xml) and specs to permanently guard against regressions.

• Updates developer setup to make Python-based parsers reproducible:

  • Adds .venv/ workflow for Python dependencies (coverage.py + pytest).
  • Pins Python version via .python-version (3.12.7).
  • Updates README with clear local dev/test instructions.
  • Integrates make test with venv so local test runs are reliable.

Why

• A customer hit the overflow when uploading a large Cobertura XML; parser failed at (1..0) range.
• This ensures coverage parsing is stable for edge cases and reduces support risk.
• The developer environment was previously fragile around Python deps; now it’s simple and reproducible.

Testing

• Added new fixtures under spec/fixtures/cobertura/.
• Extended cobertura_parser_spec.cr to assert parsing succeeds on those fixtures.
• Full suite: ✅ 123 examples, 0 failures.

Release note

Fixes Cobertura parser overflow for empty/zero-line classes, adds regression tests, and improves developer setup. Safe to cut a new release after merge.

[coveralls-official]

Pull Request Test Coverage Report for Build rc-aarch64-17813643176

Details

• 13 of 15 (86.67%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-3.4%) to 90.633%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/coverage_reporter/parsers/cobertura_parser.cr	8	10	80.0%

Totals
Change from base Build 12017105939:	-3.4%
Covered Lines:	1340
Relevant Lines:	1457

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💛 - Coveralls