feat: add eviction callback in LRU cache

[DracoLi]

Why this should be merged

Adds an optional onEvict callback to the LRU cache to allow resource cleanup when entries are evicted.

x/blockdb (#4027) uses the LRU cache to store file descriptors, so this allows blockdb to properly close the file descriptors when an entry is evicted.

How this works

• Adds SetOnEvict(callback) method to set an optional cleanup function
• The callback is called whenever an entry is evicted (during Put, Evict, or Flush)
• Existing code continues to work unchanged

How this was tested

Unit tests added

Need to be documented in RELEASES.md?

Don't think so

[Copilot]

Pull Request Overview

Adds an optional eviction callback (onEvict) to the LRU cache API and verifies its behavior in unit tests.

• Introduces SetOnEvict to register a callback invoked before entries are removed.
• Updates Put, Evict, and Flush methods to trigger the callback on eviction.
• Adds tests for eviction and flush scenarios with callbacks.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cache/lru/cache.go	Added onEvict field, SetOnEvict and invocation in Put, Evict, and Flush.
cache/lru/cache_test.go	Added tests for flush and put evictions using onEvict.

Comments suppressed due to low confidence (1)

cache/lru/cache.go:73

• There’s no unit test covering the Evict method when an onEvict callback is set. Consider adding a test to ensure onEvict is called when an entry is explicitly evicted via Evict().

	c.lock.Lock()