Add LRUCache test for get() recency promotion.

[anxkhn]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Test coverage

Motivation and Context

LRUCache.get(_:) is a real LRU read: on a hit it refreshes recency by moving
the node to most-recently-used before returning
(Sources/SocketForwarder/LRUCache.swift, the listRemove + listInsert(after: tail)
pair in get). The existing LRUCacheTest.testLRUCache never observes that
promotion, though: every get call sits at the end of the test purely to read
final values after all put-driven evictions are done. No test reads an existing
key and then forces an eviction to check that the just-read key survived.

Because of that, the recency update in get is a mutation survivor: removing it
(making get a plain lookup that no longer counts as use) keeps every current
assertion green, since put already promotes on insert/replace and the test only
relies on put-driven ordering. A regression that stopped treating reads as use
would ship undetected.

This is a test-only change: it adds one focused test and leaves production code
untouched.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

testGetPromotesToMostRecentlyUsed fills a size-3 cache with foo, bar, baz,
reads foo (promoting it to most-recently-used), then inserts qux and asserts
the evicted entry is ("bar", "2") rather than ("foo", "1"), that foo
survived, and that bar is gone. It fails if the promotion in get is removed
and passes with it.

Run:

swift test --filter LRUCacheTest

Both testLRUCache and the new test pass. To confirm it closes the gap, removing
the two promotion lines in get(_:) leaves testLRUCache green but makes the new
test fail, then reverting restores both to green.