feat: expose index segments in Python scanner

[hfutatzhanghb]

Summary

Expose vector index segment selection through the Python scanner API. This adds an index_segments argument to LanceDataset.scanner(...) and ScannerBuilder, accepts str and uuid.UUID values in Python, parses them into Rust Uuids in the PyO3 binding, and forwards them to the existing Rust Scanner::with_index_segments support.

Why

Rust already supports restricting vector search to selected index segments. Python users creating distributed index segments need the same control from ds.scanner(...), similar to the existing fragments argument.

Validation

• cargo check --manifest-path python/Cargo.toml -p pylance
• git diff --check --cached

I did not complete the targeted pytest run because it triggered a local pylance rebuild and was stopped to avoid generating additional large target artifacts.

[claude]

Claude Code Review

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[hfutatzhanghb]

@Xuanwo Hi, could you please help review this PR when have free time? Thanks a lot