InMemoryCacheClient: SetIfHigher/SetIfLower with a non-winning value still changes the TTL, unlike Redis #581

When the existing value wins, InMemoryCacheClient still applies expiresIn (current.WithExpiration(expiresAt) in SetIfAsync). With expiresIn: null, it clears an existing TTL. Redis's SetIfHigher.lua/SetIfLower.lua leave the key untouched and return 0.

main behaved this way before #570, which kept it on purpose to avoid an unrelated behavior change. The fix is one line (return current instead of current.WithExpiration(expiresAt)) plus a shared contract test in CacheClientTestsBase. Anyone relying on it today (for example, sliding a TTL with SetIfHigher) would need a heads-up.