fix(dsl): Make single-file rule loading consistent with directory behavior

[shivasurya]

Changes

• Return empty list instead of error for files without code analysis rules
• Add debug logging for container rule loading failures
• Add validation to ensure at least one type of rule is loaded

Testing

• Updated existing tests
• Added new test cases for container rule files
• All tests passing (300 Python + all Go tests)

[safedep]

SafeDep Report Summary

No dependency changes detected. Nothing to scan.

_{This report is generated by SafeDep Github App}

[codecov]

Codecov Report

❌ Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.14%. Comparing base (224d996) to head (a60bf5e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sast-engine/cmd/scan.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #501      +/-   ##
==========================================
- Coverage   81.19%   81.14%   -0.05%     
==========================================
  Files         113      113              
  Lines       13294    13302       +8     
==========================================
  Hits        10794    10794              
- Misses       2095     2103       +8     
  Partials      405      405              

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