Add regression tests for vec0_parse_vector_column edge cases

Add SQLITE_EMPTY tests for non-vector column inputs (primary key,
partition key, unknown types) and SQLITE_ERROR tests for zero
dimensions and empty brackets. Tighten existing error assertions
from rc != SQLITE_OK to exact expected return codes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>