fix(db): ensure transaction depth resets when rollback fails

[xiaobaZeo]

Summary

In NodeSqliteAdapter.transaction() (src/db/sqlite-adapter.ts), if a transaction encounters an error and SQLite automatically aborts the transaction before a rollback is executed, calling ROLLBACK throws an error (cannot rollback - no transaction is active). Previously, this._txDepth = 0 was placed after this._db.exec('ROLLBACK') without a finally block, causing _txDepth to remain greater than 0 on error. As a consequence, subsequent transaction() calls on the same connection falsely assumed they were inside a nested transaction and omitted BEGIN/COMMIT, degrading operations to auto-commit and violating ACID isolation.

This PR wraps the rollback and depth reset in a finally block with a committed flag, ensuring:

1. _txDepth is guaranteed to reset to 0 regardless of closure errors, commit failures, or rollback failures.
2. Any rollback error when SQLite has already aborted the transaction is safely tolerated.
3. Added unit tests in __tests__/sqlite-backend.test.ts covering both standard rollback and pre-aborted transaction scenarios.
4. Added user-facing entry in CHANGELOG.md under ## [Unreleased].

Validation

• npx vitest run __tests__/sqlite-backend.test.ts passes (4 tests passed).
• npm run build succeeds cleanly.

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