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_verify.py

@@ -2,9 +2,8 @@
 Tests for migration verification.
 """
 
-import json
 import pytest
-from unittest.mock import Mock, MagicMock
+from unittest.mock import Mock
 
 from es_ob_migration.verify import MigrationVerifier, VerificationResult