[Feature](lance) Support multi-vector columns and queries

## Parent

Tracked by #66340. Single-vector search was introduced by #65730.

## Scope

- [ ] Map List> Lance columns for multi-vector use.
- [ ] Accept multiple sub-vectors in one logical query.
- [ ] Implement Multi-vector Search.
- [ ] Define aggregation, scoring, filtering, Top-K, and _distance semantics.
- [ ] Do not implicitly expand the scope to joint search across multiple vector columns.

## Completion criteria

- [ ] FE type analysis, serialization, and BE execution are implemented.
- [ ] Tests cover supported element types, dimensions, nullability, and scoring behavior.
- [ ] User documentation describes the query format and result semantics.
- [ ] Representative multi-vector datasets and indexes are verified.

Start with the parent issue #66340 and the single-vector search work in #65730. Trace the FE type analysis, serialization, and BE execution paths, then review how supported element types, dimensions, nullability, scoring, filtering, Top-K, and _distance are tested. Done means multi-vector queries, representative datasets and indexes, and user documentation cover the stated semantics.

Written by the indexing model from the issue text.