flush_group optimization

[nickchomey]

removed redundant loop and unnecessary variable.

added optional capability to only clear a group from apcu, rather than whole thing

dont wipe $this->not_in_persistent_cache at end

[Copilot]

Pull request overview

This PR improves group flushing behavior in the SQLite object-cache drop-in by making APCu eviction more targeted and simplifying the in-memory group flush logic.

Changes:

• Add an optional $group_prefix parameter to apcu_clear_cache() to support clearing a subset of APCu entries.
• Update flush_group() to clear only the relevant APCu entries and remove a redundant “collect then unset” loop.
• Stop resetting $this->not_in_persistent_cache at the end of flush_group().

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[nickchomey]

copilot made good points about the double loop. i reverted that change, but some of the changes still valid