test(proto): add unit tests for metadata descriptors

[ijsong]

What this PR does

• Add tests for StorageNodeMetadataDescriptor and LogStreamReplicaMetadataDescriptor
• Test methods: ToStorageNodeDescriptor, GetLogStream, Head, and Tail
• Ensure proper coverage for edge cases and nil values

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.48%. Comparing base (a0af1e1) to head (2f9a7c7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## refactor_snmd_getlogstream #919 +/- ## ============================================================== + Coverage 67.04% 67.48% +0.43% ============================================================== Files 179 179 Lines 21607 21607 ============================================================== + Hits 14487 14581 +94 + Misses 6357 6252 -105 - Partials 763 774 +11 ```

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