fix: close open file handles in expert_vault.py (#25)

[HeaTTap]

closes #25

wraps file open calls in expert_vault.py inside with blocks to ensure file handles are properly closed on exit or on decryption errors

changes:

• updated try_unlock to use with blocks when reading expert_meta.json and expert.enc
• updated expert_count to use with block when reading expert_meta.json
• added unit tests in tests/test_expert_vault.py covering file handle closure, missing files, and unlock lifecycle

testing:

• pytest -q passed all 32 tests
• ruff check passed

[CyberSunil]

Thanks for this, and for the tests! The with-block cleanup is a good idea. The problem is this branch was created before #3 was merged, so right now it would revert the integrity-check work from #3 (it removes EXPECTED_ENC_SHA256 / EXPECTED_META_SHA256, the hashlib/hmac imports, and VaultLoadError). Could you rebase onto the latest main and re-apply just the file-handle with blocks on top of the current code? Once it's rebased with no conflicts and CI is green, I'll review and merge.

Quick steps:

git remote add upstream https://github.com/CyberSunil/LLMVault.git # once
git fetch upstream
git checkout fix-expert-vault-open-file-handles
git rebase upstream/main # resolve conflicts, KEEP main's integrity checks, add your with-blocks
git push --force-with-lease