Fix thread-safety issue in LogRecordSharedPool.Rent()

[kimjaejung96]

Fixes #6233

Changes

This PR fixes a thread-safety issue in LogRecordSharedPool.Rent() where the same LogRecord instance could be rented to multiple threads concurrently under high contention.

• Fixed Race Condition: In Rent(), when TryRentCoreRare fails (due to a race), changed the behavior to break the loop and create a new LogRecord instance. Previously, it would continue and retry with a new index, which could lead to a duplicate rental.
• Optimized Calculation: Extracted slotIndex calculation to avoid redundant modulo operations.
• Added Regression Test: Added RentShouldNeverReturnSameInstanceConcurrently in LogRecordSharedPoolTests.cs to verify the fix.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)