fix(index): align distributed scalar index parameters

[ddupg]

Summary

index_type accepts either a string or IndexConfig, but the two forms previously behaved differently:

• String inputs required canonical uppercase names and used separate driver-side checks. IndexConfig had no matching preflight check, so unsupported inputs could reach the legacy or worker path instead of failing locally.
• "FTS" worked as a string alias, but IndexConfig("fts", parameters) was passed to Core as the literal fts plugin name. Core expects inverted, so that form failed.
• Field validation ran only for strings and covered only part of Core's accepted field shapes. IndexConfig skipped driver-side validation, so equivalent requests could accept different fields or fail at different stages.

This change gives both forms one driver-side contract: normalize the logical type, preserve the caller-owned IndexConfig, map FTS configs to inverted, validate fields against Core's rules, and reject unsupported types before Ray workers start.

Validation

• tests/test_vector_index_options.py plus three focused integration tests — 36 passed
• ruff check lance_ray/index.py tests/test_vector_index_options.py tests/test_distributed_indexing.py
• git diff --check