fix: add backward compatibility for Qdrant legacy collection detection

Implement intelligent legacy collection detection to support multiple
naming patterns from older LightRAG versions:
1. lightrag_vdb_{namespace} - Current legacy format
2. {workspace}_{namespace} - Old format with workspace
3. {namespace} - Old format without workspace

This ensures users can seamlessly upgrade from any previous version
without manual data migration.

Also add comprehensive test coverage for all migration scenarios:
- Case 1: Both new and legacy exist (warning)
- Case 2: Only new exists (already migrated)
- Backward compatibility with old workspace naming
- Backward compatibility with no-workspace naming
- Empty legacy collection handling
- Workspace isolation verification
- Model switching scenario

Testing:
- All 15 migration tests pass
- No breaking changes to existing tests
- Verified with: pytest tests/test_*migration*.py -v

tests/test_migration_complete.py

@@ -13,12 +13,10 @@ Tests are implemented for both PostgreSQL and Qdrant backends.
 """
 
 import pytest
-import asyncio
 import numpy as np
 from unittest.mock import MagicMock, patch, AsyncMock
 from lightrag.utils import EmbeddingFunc
 from lightrag.kg.qdrant_impl import QdrantVectorDBStorage, _find_legacy_collection
-from lightrag.kg.postgres_impl import PGVectorStorage
 
 
 # ============================================================================