KAFKA-16158: Cleanup usage of TimestampedBytesStore interface

[gabriellefu]

1. KeyValueToTimestampedKeyValueByteStoreAdapter now implements TimestampedBytesStore, fixing the inconsistency where it stored <timestamp><value> data without advertising the interface.
2. Reordered StateManagerUtil.converterForStore to check the wrapped chain for adapters first, returning identity() for the KV adapter before the isHeadersAware/isTimestamped checks.
3. Removed the now-redundant StoreQueryUtils.isAdapter() helper and its two call sites (CachingKeyValueStore, StoreQueryUtils.deserializeValue), since WrappedStateStore.isTimestamped() now returns true for the adapter.
4. Rewrote StateManagerUtilConverterTest to use real store-builder chains instead of mocks, giving genuine round-trip regression coverage for KAFKA-16141 (in-memory retains timestamp 42, persistent-plain uses dummy -1).