fix(python): preserve index details in python index metadata

[Xuanwo]

This PR fixes Python index metadata round-tripping so Index.index_details is preserved when metadata crosses the Python/Rust boundary.

Without this, segment metadata returned from create_index_uncommitted(...) can lose index_details when passed back through Python, which breaks merge_existing_index_segments(...) and makes committed index metadata less faithful.

[github-actions]

PR Review

Clean refactoring that eliminates the uses_segment_commit_path branch and routes all index types through the same execute_uncommitted → segment builder → commit pipeline. The identity plan shortcut for single-segment inputs is a good way to generalize without adding overhead for non-vector indices.

One thing to consider

is_vector_segment relies on type URL suffix matching (create.rs new helper):

fn is_vector_segment(segment: &IndexMetadata) -> bool {
    segment.index_details.as_ref()
        .is_some_and(|details| details.type_url.ends_with("VectorIndexDetails"))
}

This works today but is fragile — if the protobuf type URL format changes or a new vector index type is added with a different message name, this will silently misclassify segments and hit the multi-input rejection error. Consider matching on an enum/variant from IndexType or the protobuf full name constant instead, if one is available. If not, a comment noting the coupling would help future readers.

Looks good otherwise

• Identity plan logic in plan() and build() is consistent and correct.
• target_segment_bytes == 0 validation is a nice addition.
• Test coverage for scalar identity build, multi-input scalar rejection, and scalar commit metadata is thorough.
• Terminology updates (delta → segment) are consistent throughout.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.19497% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index/create.rs	91.19%	11 Missing and 3 partials ⚠️

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