🧪 [testing improvement] Add unit tests for build_index in index-vault.py

[masuda-so]

🎯 What: The testing gap for the build_index function inside scripts/index-vault.py was addressed. This complex function interacts with both the filesystem and an internal SQLite database, requiring tests to validate behavior like note scanning, directory exclusions, metadata extraction, and incremental update logic.

📊 Coverage:
The newly added suite in scripts/tests/test_index_vault.py tests several critical scenarios:

• Basic Indexing: Scanning valid directories, ignoring explicitly skipped ones, and verifying metadata insertion.
• Incremental Indexing: Tests incremental=True behavior, capturing state changes for unmodified, updated, added, and deleted files.
• Non-incremental Sync: Ensures files removed from the filesystem are dropped from the database when doing a full rebuild.
• Permissions Failure Resilience: Handles files throwing OSError (e.g. unreadable permission setup) gracefully.
• Content Extraction Logic: Mocks a note with full frontmatter structure, nested links, and markdown callouts to ensure title, tags, summaries, and lengths are accurately processed.

✨ Result: Test coverage improved specifically on index-vault's core functionality. Tests execute securely in isolated temporary directories ensuring zero side effects, making future refactors much more confident and secure against bugs.

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PR created automatically by Jules for task 3442137096001501526 started by @masuda-so

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