Fix HTML autotrim bug - respect directory boundaries when computing common prefix

[Copilot]

The HTML autotrim feature was incorrectly computing common prefixes that could end in the middle of directory names, causing partial directory names to be stripped from file paths.

Problem

When multiple SARIF results had file paths with similar prefixes that didn't align with directory boundaries, the autotrim logic would compute a common prefix that cut through directory names. For example:

Input paths:

tools/bin/RedacteA/RedacteB/RedacteB.deps.json
tools/bin/RedacteC/RedacteD/RedacteD.deps.json  

Before fix: Common prefix computed as tools/bin/Redacte, resulting in broken output:

ools/bin/RedacteA/RedacteB/RedacteB.deps.json:953
ools/bin/RedacteC/RedacteD/RedacteD.deps.json:960

After fix: Common prefix computed as tools/bin/, resulting in correct output:

RedacteA/RedacteB/RedacteB.deps.json:953
RedacteC/RedacteD/RedacteD.deps.json:960

Solution

Modified the init_path_prefix_stripping method in sarif_file.py to ensure that computed common prefixes always end at directory boundaries (slashes). The fix:

1. Finds the common prefix between paths as before
2. Locates the last directory separator (slash) in the common prefix
3. Truncates the prefix to end at that directory boundary
4. If no directory separator is found, disables autotrim for that case

Testing

Added comprehensive tests covering:

• The original bug case with partial directory name prefixes
• Paths with no common directory prefix (autotrim disabled)
• Paths with proper common directory prefixes (autotrim works correctly)
• Edge cases with deeply nested paths

All existing tests continue to pass, ensuring no regressions.

Fixes #85.

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[LensPlaysGames]

fixes #110

oops. lgtm???