fix: accumulate all username removals instead of only preserving the last one

[zkoppert]

Pull Request

Proposed Changes

Fixes the username removal loop in main() so that removing multiple users from a CODEOWNERS file works correctly. Four issues are addressed:

1. Accumulation bug (issue bug: only last username removal preserved when multiple users need removal from CODEOWNERS #380): The loop re-derived each removal from the original content, so only the last removal survived. Now initializes codeowners_file_contents_new = codeowners_decoded before the loop and replaces from that accumulator each iteration.

2. Substring matching bug (discovered during self-review): bytes.replace(b"@bob", b"") corrupts @bobsmith into smith. Replaced with re.sub() using a lookahead assertion (?=\s|$) so only exact handles are matched. The accumulation fix made this pre-existing issue worse because corrupted content now persists across iterations instead of being overwritten.

3. Whitespace cleanup, scoped to changed lines (discovered during self-review, refined per review feedback): After removing a username, leftover double spaces and trailing whitespace are cleaned up. Uses [ \t]{2,} to collapse horizontal whitespace and [ \t]+(?=\r?$) lookahead to strip trailing whitespace without consuming \r in CRLF files. Cleanup is scoped to only lines where a username was actually removed, preserving intentional alignment on unrelated lines.

4. Extracted helper functions (per review feedback): remove_username_from_content() and cleanup_whitespace() reduce nesting depth in main() and make the removal/cleanup logic directly testable.

Closes #380

Testing

• test_multiple_username_removals_are_cumulative - verifies removing @alice and @bob preserves all removals
• test_username_removal_does_not_corrupt_similar_names - verifies removing @bob leaves @bobsmith intact
• test_username_removal_cleans_up_whitespace - verifies no double spaces left after removal
• test_username_removal_handles_crlf_line_endings - verifies cleanup works with \r\n endings without consuming \r
• test_whitespace_cleanup_scoped_to_changed_lines - verifies unrelated lines with intentional alignment are not modified
• All 68 tests pass, 96% code coverage
• make lint clean (pylint 9.99/10, mypy, flake8, isort, black all pass)

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request
• run make lint and fix any issues that you have introduced
• run make test and ensure you have test coverage for the lines you are introducing

[jmeridth]

Reviewed the PR — the core accumulation fix and substring matching fix are both correct and well-motivated. Two issues in the whitespace cleanup block worth addressing before merge.

[zkoppert]

Great catches on both of those!! Addressing now. 🏃🏻