fix: make vector distance schema nullable

[BubbleCal]

Bug Fix

• What is the bug?
  • Vector search result schemas disagreed on the _distance column nullability. The public scanner and KNN schemas already expose _distance as nullable, while VECTOR_RESULT_SCHEMA and a mem-wal vector path still marked it as non-nullable.
• What issues or incorrect behavior does the bug cause?
  • The mismatch can create inconsistent schemas across vector search execution paths and tests, even though callers should see a single nullable _distance contract.
• How does this PR fix the problem?
  • Updates the remaining vector result schema definitions and related test fixtures to mark _distance as Float32 nullable.
  • Updates the VectorIndex schema documentation to match the nullable contract.

Validation

• rg check for remaining explicit _distance Float32 nullable=false definitions under rust/lance and rust/lance-index
• cargo fmt --all --check
• cargo test -p lance --lib dataset::mem_wal::scanner::exec::filter_stale::tests:: --no-fail-fast
• git diff --check

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/dataset/mem_wal/memtable/scanner/exec/vector.rs	0.00%	1 Missing ⚠️

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[claude]

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[justinrmiller]

lgtm