fix: continue LRU eviction after removals

[siriookie]

#1019

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Note

Low Risk
Small, localized fix to client-side LRU bookkeeping in Update, covered by a new regression test; no auth or persistence impact.

Overview
Fixes LRU eviction in Update when a newly stored value pushes c.size over c.max. The eviction walk from the list front now saves next := ele.Next() before Remove, then continues with ele = next, instead of calling Next() on a removed node (which could stop eviction early and leave stale entries).

Adds Cache Evict Multiple Entries in lru_test.go: a cache sized for two small entries must evict both when a slightly larger third entry is updated in, asserting final size, list length, and that keys 0/1 are gone while the new key remains.

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[rueian]

Thanks @siriookie