fix: fix thread-safe problem for file store path factory

[lucasfang]

Purpose

N/A

This change fixes a thread-safety issue in FileStorePathFactory cache access paths.

• Protect cache reads/writes in ToBinaryRow and GetPartitionString with std::shared_mutex.
• Use read lock for fast-path cache hits and write lock for insertions, with double-check under write lock.
• Update API note to reflect that GetPartitionString is now thread-safe for concurrent calls.

Tests

UT:

• FileStorePathFactoryTest.TestConcurrentToBinaryRowAndGetPartitionString
  • Runs 8 threads with 500 iterations per thread.
  • Concurrently calls ToBinaryRow and GetPartitionString and validates deterministic outputs.

Existing UTs in the same suite still pass for non-concurrent paths.

IT:

• N/A

API and Format

No API behavior change for external callers, and no storage format/protocol change.

Internal update only:

• Added cache_mutex_ (std::shared_mutex) to guard in-memory caches.
• Updated method comment for thread-safety semantics.

Documentation

No new feature introduced. No documentation update required.

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Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[lxy-9602]

+1

[zjw1111]

+1