Add OCC validation and conflict detection tests

[bootjp]

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[gemini-code-assist]

Summary of Changes

Hello @bootjp, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the key-value store's transaction management by integrating robust Optimistic Concurrency Control (OCC) validation directly into the Finite State Machine (FSM). The changes ensure that concurrent writes are properly checked for conflicts against existing data, preventing stale updates and maintaining data integrity. Comprehensive unit tests have been added to validate the new conflict detection mechanisms.

Highlights

• Optimistic Concurrency Control (OCC) Validation: The kvFSM.validateConflicts function has been updated to implement actual OCC conflict detection. It now checks if a transaction's startTS is older than the LatestCommitTS for any key it attempts to modify, returning an error if a conflict is detected.
• New OCC Test File: A new test file, kv/fsm_occ_test.go, has been added to thoroughly test the new OCC validation logic, including scenarios for detecting stale writes and ensuring the Apply method correctly returns errors on conflict.

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[gemini-code-assist]

Code Review

This pull request introduces OCC conflict validation by implementing the validateConflicts function and adding corresponding tests. The implementation correctly detects write conflicts, and the new tests in kv/fsm_occ_test.go effectively verify this functionality.