Add LRU cache eviction to CombinePerKeyPrecombineOperator

[junaiddshaukat]

Summary

This PR implements LRU (Least Recently Used) cache eviction for CombinePerKeyPrecombineOperator, addressing the TODO at line 487 of operators.ts.

Changes

1. Added touchKey() method - Moves accessed keys to the end of the Map (most recently used position)
2. Added flushLRU() method - Evicts entries from the front (least recently used) until cache is under limit
3. Updated process() method - Each key access now updates its LRU position

Technical Details

JavaScript Maps preserve insertion order, so we implement LRU by:

• On access: delete and re-insert the key (moves to end)
• On eviction: iterate from front (oldest entries first)

Related Issues

• Addresses TODO comment in operators.ts:487
• Follow-up from discussion in Add LRU cache eviction to CachingStateProvider #37214

Testing

• All 297 existing tests pass
• Prettier check passes

Fixes: #37465

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[gemini-code-assist]

Summary of Changes

Hello @junaiddshaukat, 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 CombinePerKeyPrecombineOperator by integrating a robust LRU cache eviction mechanism. This change addresses a long-standing TODO, improving the operator's memory management and performance by intelligently removing less frequently accessed data when the cache capacity is reached. The implementation leverages native JavaScript Map behavior to maintain the LRU order efficiently.

Highlights

• LRU Cache Eviction: Implemented Least Recently Used (LRU) cache eviction for the CombinePerKeyPrecombineOperator to efficiently manage its internal state when the number of keys exceeds a defined limit.
• New touchKey() Method: Introduced a touchKey() private method that moves an accessed key to the end of the internal Map, effectively marking it as most recently used by leveraging JavaScript Map's insertion order preservation.
• New flushLRU() Method: Added a flushLRU() method responsible for evicting the least recently used entries from the cache (those at the front of the Map) until the cache size is within the specified limit.
• Updated process() Logic: The process() method was updated to utilize the new touchKey() method for every key access, ensuring that the LRU order is maintained. It also now calls flushLRU() instead of the previous random flush logic when the cache size limit is exceeded.

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[junaiddshaukat]

Hi @damccorm, added LRU to PrecombineOperator. Please have a look when you got time, Thanks.

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