feat: create vector indices without training

[burlacio]

Summary

Enable create_index(..., train=False) for vector indices,
completing the empty-index story that already worked for
scalar/FTS indices.

Previously this path hit todo!("create empty vector index when train=false") and panicked.

The work touches three things:

• rust/lance/src/index/create.rs — removes the todo!()
  and lets the existing build_empty_vector_index helper handle
  the train=false branch.
• rust/lance/src/index.rs — empty (definition-only) indices
  have no on-disk files, so the stats and vector-detection paths
  needed adjustments:
  • collect_regular_indices_statistics now short-circuits when
    fragment_bitmap is empty and returns {} instead of trying to
    open a non-existent index file.
  • is_vector_index detection prefers index_details metadata
    over file-based probing, since empty indices have no
    INDEX_FILE_NAME to inspect. The file-based path is kept as a
    fallback for older indices.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index.rs	88.88%	1 Missing and 1 partial ⚠️

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