Create pipeline to validate DBX JSON certificate references

[Copilot]

This PR implements a validation pipeline to ensure that DBX JSON files reference certificate files that actually exist in the PreSignedObjects/DBX/Certificates folder.

Problem

When new DBX JSON files are created, the internal certificate names referenced in the JSON don't always match the external filenames in the Certificates folder, and there was no validation to catch these mismatches. For example, the current dbx_info_msft_06_10_25.json references WindowsProduction2011.cer but the actual file is named MicWinProPCA2011_2011-10-19.der.

Solution

Added a new validation script and CI pipeline step that:

1. Finds the latest DBX JSON file - Automatically locates dbx_info_msft_<date>.json files
2. Validates certificate references - Checks that all certificates listed in the "certificates" array actually exist in the Certificates folder
3. Provides clear error messages - Shows exactly which certificates are missing and lists available files for debugging
4. Handles edge cases - Gracefully handles missing certificates sections, malformed JSON, etc.

Changes Made

New Files

• scripts/validate_dbx_references.py - Main validation script with CLI interface
• scripts/test_validate_dbx_references.py - Comprehensive unit tests (7 test cases)

CI Integration

• Added validation step to .github/workflows/prepare-binaries.yml after unit tests
• Pipeline will now fail on PR/release if certificate references are invalid

Other

• Fixed .gitignore to properly exclude __pycache__ directories
• Removed accidentally committed cache files

Testing

# Current mismatch is detected
$ python scripts/validate_dbx_references.py PreSignedObjects/DBX
ERROR: Certificate file 'WindowsProduction2011.cer' referenced in JSON but not found in PreSignedObjects/DBX/Certificates
INFO: Available certificate files:
INFO:   - MicWinProPCA2011_2011-10-19.der

# All unit tests pass
$ pytest scripts/test_validate_dbx_references.py
7 passed in 0.01s

The validation script successfully detects the existing mismatch and will prevent similar issues in the future through automated CI checks.

Fixes #224.

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[Flickdm]

This will require that PR #223 goes in first as currently the Pipeline is failing