refactor: remove index segment builder

[Xuanwo]

Remove the deprecated IndexSegmentBuilder API and route staged index publishing through the existing create_index_uncommitted, merge_existing_index_segments, and commit_existing_index_segments workflow.

IndexSegmentBuilder was kept in the codebase after distributed vector indexing moved to the flatter merge-and-commit workflow. That leftover API created two competing ways to describe the same lifecycle, which made the codebase and bindings harder to reason about and caused confusion around whether callers should plan/build segments or just merge and commit existing segment metadata.

This PR removes the parallel plan/build surface from Rust, Python, and Java bindings while keeping direct uncommitted segment commit support. FTS staged segments now finalize during commit metadata construction, so single-source staged FTS outputs no longer need a separate builder step before publishing.

[codecov]

Codecov Report

❌ Patch coverage is 97.91667% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index.rs	93.75%	0 Missing and 1 partial ⚠️

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