patterns: clean up TODOs, move is_include_cmd to IECommand property, fixes #9442

[mr-raj12]

Description

Address the TODO items in src/borg/patterns.py as discussed in #9442:

1. recurse_dir_default: Replaced the misleading TODO with a comment explaining why it must stay True as include patterns inside excluded directories (+ inside -) depend on it.

2. is_include_cmd: Moved from a dict on PatternMatcher to an is_include property on IECommand and now the matcher calls cmd.is_include instead of doing a dict lookup.

3. RootPath validation: Added a warning when the root path doesn't start with /, so backups don't fail due to a typo.

4. command_recurses_dir: Replaced cmd not in [IECommand.ExcludeNoRecurse] with explicit if/raise branches using is comparisons. Now raises ValueError for unexpected commands like RootPath or PatternStyle.

Tests updated to use real IECommand values and cover the new code paths.

Checklist

• PR is against master
• New code has tests and docs where appropriate
• Tests pass
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.36%. Comparing base (9d5687d) to head (60596bd).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/patterns.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9542      +/-   ##
==========================================
- Coverage   83.39%   83.36%   -0.04%     
==========================================
  Files          87       87              
  Lines       15469    15481      +12     
  Branches     2321     2325       +4     
==========================================
+ Hits        12901    12906       +5     
- Misses       1818     1821       +3     
- Partials      750      754       +4     

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[ThomasWaldmann]

LGTM!