test(backend): add unit tests for PartitionState type

[refcell]

Summary

Add comprehensive unit tests for the PartitionState type in the backend crate.

Changes

• crates/storage/backend/src/partition.rs: Added 17 new unit tests covering:
  • new() and default() return uninitialized state
  • initialize() state transitions (success from uninitialized, fail from ready/closed)
  • get() returns reference when ready, errors otherwise
  • get_mut() allows mutation when ready, errors otherwise
  • close() state transitions (success from ready, errors otherwise)
  • is_ready() accurately reflects state throughout lifecycle
  • Debug trait implementation

Verification

• All 17 tests pass
• Clippy clean
• Comprehensive state machine coverage for partition lifecycle