Add targeted check --junitxml test coverage for XML output and failure reporting

[Copilot]

The review feedback requested explicit coverage for the new --junitxml mode in the check CLI path. This update adds focused tests in tests/test_check.py to assert report generation and failure serialization for invalid rules.

• Test coverage added

  • Added test_check_junitxml_created_and_well_formed to verify:
    • report file is created
    • XML parses successfully
    • root structure is valid (testsuite or testsuites)
  • Added test_check_junitxml_reports_failures_for_invalid_rules to verify invalid rules emit <failure> entries.

• Branch-safe guard for option availability

  • Added a lightweight helper that skips these tests when check does not expose a junitxml parameter (keeps tests scoped to the reviewed feature without introducing unrelated failures in branches lacking the option).

def skip_if_junitxml_unavailable():
    if "junitxml" not in {param.name for param in check.params}:
        pytest.skip("--junitxml option is not available in this branch")

def test_check_junitxml_reports_failures_for_invalid_rules(tmp_path):
    runner = CliRunner()
    skip_if_junitxml_unavailable()
    report_path = tmp_path / "check-report.xml"
    result = runner.invoke(check, ["--junitxml", str(report_path), "tests/files/invalid"])
    root = ET.parse(report_path).getroot()
    assert result.exit_code == 1
    assert len(root.findall(".//failure")) > 0