Fix S3 vectors knowledge base validation

[gabrielbressan-tfy]

Summary

• add get_dimension() to the S3 vectors handler so knowledge base creation validates dimensions via S3 index metadata instead of a broad SELECT
• treat S3 Vectors ConflictException as an existing index when if_not_exists=True
• add unit coverage for get_dimension, existing-index retries, and preserving strict SELECT behavior

Tests

• docker exec dev-mindsdb python -m unittest tests/unit/handlers/test_s3vectors_handler.py