Add test coverage for malformed Cobertura XML files

[coderabbitai]

Description

Tests for malformed lcov files exist in the generate-coverage test suite, but similar coverage for Cobertura XML is missing. Adding tests for cases like missing elements or invalid XML structure would improve error handling in the Cobertura parser.

Current State

• ✅ Tests exist for malformed lcov files (e.g., test_lcov_malformed_file)
• ❌ No equivalent tests for malformed Cobertura XML files

Suggested Tests

• Invalid XML structure
• Missing required elements
• Malformed attributes
• Empty or corrupted XML content

References

• PR: Refactor test modules to rely on real Typer integration #85
• Comment: Refactor test modules to rely on real Typer integration #85 (comment)

Requested by: @leynos