Add RocksDBPartitionedIndexBackend into DynamicBucketAssignFunction

### Task Description

**What needs to be done:**
Wire RocksDBPartitionedIndexBackend into DynamicBucketAssignFunction — Behind the disabled-by-default flag, allow DynamicBucketAssignFunction to construct/hold the new backend instead of RecordLevelIndexBackend. Still no bootstrap — cache starts empty and misses fall through to MDT (existing fallback path), so this is safe to land and flag-gated.

**Why this task is needed:**

### Task Type

Code improvement/refactoring

### Related Issues

**Parent feature issue:** (if applicable )
**Related issues:**
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