Support RDB_TYPE_HASH_2 (Valkey 9) in DumpRdbValueVisitor

[otheng03]

Summary

Adds parsing support for RDB_TYPE_HASH_2 (opcode 22, introduced in Valkey 9 / RDB v80), which encodes a hashtable with field-level expirations. Wire format per entry is [field, value, ttl_ms], distinct from RDB_TYPE_HASH_METADATA (Redis 7.4) which uses [ttl, field, value] with a leading minExpire.

Changes

• DefaultRdbVisitor#applyHash2 — new override that emits KeyStringValueTTLHash with valueRdbType = RDB_TYPE_HASH_2, mirroring applyHashMetadata.
• DumpRdbValueVisitor#applyHash2 — two branches:
  • When the target version is < 80, downgrade to RDB_TYPE_HASH and drop per-field TTLs so the dump is restorable on pre-Valkey-9 servers.
  • Otherwise, passthrough using the SkipRdbParser + RawByteListener pattern (no re-encoding).
• Tests in DumpRdbVisitorTest:
  • testHash2Passthrough — asserts re-parsed dump preserves keys, values, and TTLs.
  • testHash2DowngradeToHash — runs DumpRdbVisitor(replicator, 79) and asserts the dump comes through as RDB_TYPE_HASH with TTLs stripped.
• src/test/resources/dump-hash2.rdb generated against a local Valkey 9 instance (commands documented in a comment above testHash2Passthrough).

Notes

• I added applyHash2 to DefaultRdbVisitor for now. Wondering if it's time to split the RDB visitor into Redis and Valkey variants — happy either way, just flagging. Thoughts welcome!
• For the version < 80 case I went with silent downgrade to RDB_TYPE_HASH (dropping per-field TTLs) to keep things compatible with older targets — same approach applyHashMetadata already takes. An alternative would be to throw so the caller knows TTLs were lost. I leaned toward compatibility but I'm not sure it's the right call. What do you think?
• Bumped the version to 3.12.0 as a placeholder — let me know if you'd prefer something different.

[leonchen83]

This PR is awesome — I’ve gone ahead and merged it.

I have an idea: now that this is in place, we could also support format conversion between Valkey and Redis (Valkey → Redis and Redis → Valkey). For example, converting HashMetadata to Hash_2, or Hash_2 back to HashMetadata. It’s just an initial thought for now — after the merge, I’ll spend some time thinking through how we might approach this.

In that case, migrating from Valkey to a newer version of Redis wouldn’t require dropping TTL information anymore.And the reverse would work as well.