test(cargo-heather): add unit tests for missed mutations

[Vaiz]

Add unit tests to catch 7 mutations that were previously missed by cargo-mutants, causing nightly CI failures since 2026-05-15.

Tests added:

• comment.rs: is_cargo_script inner attribute rejection, missing frontmatter, valid script detection; is_header_comment_line doc comment exclusion (/// and //!)
• error.rs: HeatherError::source() returns io::Error for FileRead variant and None for others
• license.rs: supported_licenses() contains known IDs and returns sorted results

Fixes #27

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.4%. Comparing base (b056994) to head (afe2512).

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #30     +/-   ##
=======================================
+ Coverage   87.2%   89.4%   +2.1%     
=======================================
  Files         15      15             
  Lines        707     756     +49     
=======================================
+ Hits         617     676     +59     
+ Misses        90      80     -10     

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[Copilot]

Pull request overview

Adds targeted unit tests in cargo-heather to cover behaviors that were previously missed by mutation testing (cargo-mutants), addressing nightly CI failures reported in #27.

Changes:

• Added unit tests around cargo-script detection and header comment classification (excluding Rust doc comments).
• Added unit tests verifying HeatherError::source() behavior for FileRead vs non-I/O variants.
• Added unit tests ensuring supported_licenses() is non-empty, contains known SPDX IDs, and returns sorted output.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/cargo-heather/src/comment.rs	Adds unit tests for cargo-script heuristics and doc-comment exclusion in header detection.
crates/cargo-heather/src/error.rs	Adds unit tests validating Error::source() behavior across error variants.
crates/cargo-heather/src/license.rs	Adds unit tests validating presence of known IDs and sorted output from supported_licenses().

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