KAFKA-20500: Add isolation-level reads to versioned stores

[nicktelford]

Part of the KIP-892 interactive-query isolation-level series. Versioned stores are queryable through IQv2 (VersionedKeyQuery, MultiVersionedKeyQuery) and IQv1, but had no way to honour the configured isolation level. When the underlying RocksDBStore is transactional its accessor consults the staged-write buffer, so a READ_COMMITTED query would incorrectly observe writes still in the current transaction.

This extends the readOnly(IsolationLevel) hook the other store families already have to versioned stores. Because versioned stores have no dedicated ReadOnly* parent interface, the default is added directly on VersionedKeyValueStore, and VersionedBytesStore.readOnly is covariantly narrowed so wrapper layers keep the versioned read methods. Reads in RocksDBVersionedStore — single-key latest, point-in-time, and timestamp-range — flow through LogicalKeyValueSegment views bound to a specific DBAccessor, so READ_COMMITTED bypasses the transaction buffer via the direct accessor. The metered and change-logging versioned wrappers gain matching overrides, and StoreQueryUtils dispatches versioned key queries through readOnly(isolationLevel).

Semantic tests assert that READ_COMMITTED hides staged writes while READ_UNCOMMITTED exposes them across the single-key, point-in-time, and timestamp-range read paths.

This branched off the now-merged RocksDB isolation-level read work (KAFKA-20498) and now applies directly to trunk.

🤖 Generated with Claude Code

[nicktelford]

@bbejeck