fix: return error for divergent MemWAL schemas

[lance-gatekeeper]

Summary

• replace the fallible Lance-schema expect with a contextual InvalidInput error
• add a regression test for an index column present only in the Arrow schema

Root cause

validate_index_configs checked the independent Arrow schema first and assumed that a matching Lance-schema field must exist. A caller-supplied divergent schema pair therefore reached expect and panicked at a public fallible API boundary.

Validation

• cargo test -p lance test_validate_index_configs --lib
• cargo fmt --all -- --check
• cargo clippy --all --tests --benches -- -D warnings

Fixes #8194

[lance-gatekeeper]

✅ Gate recommendation: approve.

Schema divergence is now rejected at the side-effect-free validation boundary with a contextual InvalidInput, preserving the field-ID invariant and preventing malformed inputs from panicking.