Fix missing strip_root behavior in Codebase return path

[Kaushik-Kumar-CEG]

Fixes #2985

Summary

Fixed strip_root parameter not stripping root directory from Resource paths when using cli.run_scan() programmatically.

Problem

• CLI --strip-root worked correctly
• But, API mode with return_codebase=True did not strip root from Resource paths
• This is the programmatic API mode that returns a Codebase object instead of JSON

Solution

• Rebuilt resources_by_path with stripped paths after Codebase creation
• Used strip_first_path_segment() to remove root prefix from each Resource
• Fixed Resource.parent() method to return root Resource (not empty string) for direct children of root by monkey-patching when strip_root is enabled

Changes

• src/scancode/cli.py: Added path stripping logic and parent() fix for return_codebase branch

Verification

• All existing tests pass
• Manual testing confirms root directory is now stripped from paths
• Behavior matches CLI --strip-root flag
• Resource.parent(codebase) correctly returns root Resource for direct children

Screenshots of Fix

BEFORE fix:

AFTER Fix

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)

[Kaushik-Kumar-CEG]

@JonoYang This PR is ready for review

The issue was that strip_root=True had no effect when using return_codebase=True because commoncode's Codebase ignores the parameter. The stripping only worked during JSON serialization, not on the actual Resource objects.

The fix rebuilds resources_by_path with stripped paths after Codebase creation. Additionally, it patches Resource.parent() because commoncode's implementation uses return parent_path and get_resource(parent_path) which returns early on empty strings when a direct child's parent path becomes '' after stripping.

Kindly review my fix and let me know if you have any feedback :)