fix(index): accept "RQ" when parsing QuantizationType

[jackylee-ch]

Display writes "RQ" for QuantizationType::Rabit but FromStr only accepted "RABIT", so the two never round-tripped.

IvfIndexState::serialize stores the quantization type with to_string(); deserialize reads it back with parse::<QuantizationType>(). For an IVF_RQ index that parse always failed, and since CacheCodec::deserialize turns a body error into CacheDecode::Miss, the failure was silent — the index just never reused its serialized state.

Display has to keep emitting "RQ": index_type_string builds IVF_{quantization_type} and IndexType::try_from expects "IVF_RQ". So the fix belongs on the FromStr side. "RABIT" stays accepted for headers already on disk.

The existing round-trip coverage missed this because test_ivf_index_state_roundtrip hardcodes QuantizationType::Flat and the test_prewarm_and_query_with_serializing_backend cases are PQ and HNSW_SQ only. The new #[rstest] covers every variant.

Testing

• cargo test -p lance-index --lib vector::quantizer — 6 pass; case_5_rabit fails without the one-line fix
• cargo test -p lance-index --lib vector:: — 331 pass
• cargo fmt --all -- --check, cargo clippy -p lance-index --tests --benches -- -D warnings

[lance-gatekeeper]

✅ Gate recommendation: approve.

The parser now accepts the canonical RQ spelling emitted by Display while preserving existing RABIT inputs. This restores IVF_RQ cache-state round trips without changing the persisted index identifier.

[Xuanwo]

Yes, I think this change makes sense

[jackylee-ch]

Thanks @Xuanwo