fix(dataset): fail-fast casting for columns with attached indices

[WenDing-Y]

Summary

Currently, calling alter_columns with a cast on a column that has an attached
index silently drops the index without any warning or error. This has caused
production incidents where vector search silently regressed to brute-force scan.

This PR adds a fail-fast check before any cast is performed: if a target
column has any index attached, the operation is rejected with a clear error
message that names the affected column(s), the index(es), and the required
remediation (drop_index() → cast → rebuild index).

Changes

• Added index-awareness check in alter_columns that scans cast target columns
  for attached indices
• Returns a descriptive Error::invalid_input with column names, index names,
  and remediation steps when a conflict is detected
• Updated existing tests that previously relied on silent index-drop behavior
  to explicitly drop_index() before casting
• Added new test test_alter_columns_cast_fails_with_attached_index covering
  the IVF_PQ index path: verifies the error message, schema integrity, index
  preservation, and that the cast succeeds after a manual drop

Breaking Change ⚠️

Callers that previously depended on the silent index-drop behavior must now
explicitly call drop_index() before alter_columns with a cast, then rebuild
the index afterward.

[claude]

Claude Code Review

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[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/schema_evolution.rs	96.42%	0 Missing and 1 partial ⚠️

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