Fix: remove unused imports and f-string formatting (#12935)

### What problem does this PR solve?

- Remove unused imports (Mock, patch, MagicMock, json, os,
RAGFLOW_COLUMNS, VECTOR_FIELD_PATTERN) from multiple files
- Replace f-string formatting with regular strings for console output
messages in cli.py
- Clean up unnecessary imports that were no longer being used in the
codebase

### Type of change

- [x] Refactoring

tools/es-to-oceanbase-migration/tests/test_schema.py

@@ -9,7 +9,6 @@ This module tests:
 """
 
 import json
-import pytest
 from es_ob_migration.schema import (
     RAGFlowSchemaConverter,
     RAGFlowDataConverter,