[includeconfigsource] deprecate delete_files setting

[atoulme]

Description:
Deprecate the delete_files setting. This makes the includeconfigsource stateless, removing a feature that is no longer desirable to support. It will be removed on or after April 2026.

[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.21%. Comparing base (985c37c) to head (d313ec5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...nternal/configsource/includeconfigsource/source.go	16.66%	2 Missing and 3 partials ⚠️

❌ Your project check has failed because the head coverage (33.21%) is below the target coverage (35.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7141      +/-   ##
==========================================
- Coverage   33.22%   33.21%   -0.01%     
==========================================
  Files         330      330              
  Lines       23321    23328       +7     
==========================================
  Hits         7748     7748              
- Misses      14678    14684       +6     
- Partials      895      896       +1     

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