fix: update entries already admitted to LRU2Cache

[zsewasdsdsd]

Summary

Fixes #16500.

This PR fixes an issue in LRU2Cache.put() where updating a key that has already been promoted to the main cache does not replace its existing value.

Problem

LRU2Cache uses a two-write admission policy:

1. The first put() records the key in the history cache (preCache).
2. The second put() promotes the key into the main cache.
3. Subsequent put() calls should update the existing cached value.

However, the current implementation only checks whether the key exists in preCache. Once a key has been promoted, it is removed from preCache, causing subsequent updates to be treated as new entries instead of updating the main cache.

As a result, callers may continue receiving stale cached values.

Changes

• Check whether the key already exists in the main cache before applying the two-write admission policy.
• Update existing entries while preserving the previous-value return behavior.
• Preserve the existing locking mechanism and LRU eviction order.
• Add regression tests covering promoted-entry updates, LRU ordering, and null-valued entries.

Verification

RED — Before the fix

• Reproduced the issue against the original upstream Java implementation.
• 3 expected regression failures; 1 existing-behavior check passed.

GREEN — After the fix

• All 4 standalone regression checks passed.
• Ran LRU2CacheTest using Maven on Windows with Java 21.
• Tests run: 7
• Failures: 0
• Errors: 0
• Skipped: 0
• Maven result: BUILD SUCCESS

No full-repository test suite or distributed RPC integration tests were run.

Impact

This change ensures that already-admitted cache entries can be updated correctly without changing the admission policy for new keys or introducing new dependencies.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.96%. Comparing base (ae809de) to head (d419b72).

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16503      +/-   ##
============================================
- Coverage     60.92%   58.96%   -1.97%     
  Complexity       15       15              
============================================
  Files          1953     1953              
  Lines         89278    89280       +2     
  Branches      13476    13476              
============================================
- Hits          54390    52641    -1749     
- Misses        29313    30945    +1632     
- Partials       5575     5694     +119     

Flag	Coverage Δ
integration-tests-java21	?
integration-tests-java8	?
samples-tests-java21	?
samples-tests-java8	?
unit-tests-java11	59.16% <100.00%> (-0.01%)	⬇️
unit-tests-java17	58.64% <100.00%> (-0.05%)	⬇️
unit-tests-java21	58.67% <100.00%> (+0.01%)	⬆️
unit-tests-java25	58.62% <100.00%> (+0.01%)	⬆️
unit-tests-java8	59.20% <100.00%> (+0.03%)	⬆️

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