Parent: #3401

Summary

LanceDB should define and test how default vector column discovery behaves when vector fields are nested.

Scope

• Decide whether default vector discovery recursively finds nested vector leaves or requires users to pass an explicit field path.
• Ensure explicit nested vector paths such as image.embedding work for index creation and search.
• Return a clear error when multiple candidate vector fields exist or when no default can be chosen.
• Keep Python/Rust/Node behavior aligned.

Acceptance criteria

• Explicit image.embedding vector index creation and search work end-to-end.
• Default vector search either discovers a single nested vector leaf or reports a clear actionable error.
• Tests cover nested vector leaf, multiple vector leaves, and no vector leaf.