v6: dynamic index quantization is never parsed into VectorConfig

[dudanogueira]

VectorConfig.CustomTypeAdapterFactory.read looks for the bq/pq/sq/rq keys at the top level of vectorIndexConfig. That is correct for hnsw and flat, but a dynamic index nests them one level deeper, under hnsw/flat, so the quantizer is silently dropped on read.

Verified against a live Weaviate 1.38.0 collection with a dynamic index and RQ enabled:

raw     hnsw.rq = {"bits":8,"enabled":true,"rescoreLimit":20}
typed   quantization = null

The same root cause loses Dynamic.flat's own quantizer.

Also dropped on read at the dynamic level: vectorIndexConfig.distance.

Fixing this needs a design call first: under dynamic, hnsw and flat can carry different quantizers, but VectorConfig has a single quantization() slot.

Found while working on #598, which adds getConfigAsJson as a raw-JSON escape hatch — that unblocks callers who need the complete picture, but does not fix the typed mapping.