add onEvict callback for TTLCache

[C-Pro]

Add func (c *MapTTLCache[K, V]) OnEvict(f onEvictFunc[K, V]) function to set a callback on MapTTLCache to be called on each record evicted from the cache due to TTL expiration.

[Copilot]

Pull request overview

This PR adds an onEvict callback feature to MapTTLCache that allows users to be notified when cache entries are evicted due to TTL expiration. The callback is intentionally not called for explicit Del operations, only for automatic TTL-based evictions.

Key changes:

• Added OnEvict method to set an eviction callback function
• Integrated callback invocation into the cleanup process
• Comprehensive test coverage for various eviction scenarios

Reviewed changes

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

File	Description
map_ttl.go	Adds onEvictFunc type, onEvict field to MapTTLCache, OnEvict setter method, and callback invocation in the cleanup method
ttl_test.go	Adds three comprehensive tests: basic eviction callback, verification that Del doesn't trigger callback, and partial cleanup with batched evictions

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