fix: Prioritize workspace-specific legacy collections in Qdrant migration

Why this change is needed:
The E2E test test_backward_compat_old_workspace_naming_qdrant was failing
because _find_legacy_collection() searched for generic "lightrag_vdb_{namespace}"
before workspace-specific "{workspace}_{namespace}" collections. When both
existed, it would always find the generic one first (which might be empty),
ignoring the workspace collection that actually contained the data to migrate.

How it solves it:
Reordered the candidates list in _find_legacy_collection() to prioritize
more specific naming patterns over generic ones:
  1. {workspace}_{namespace}  (most specific, old workspace format)
  2. lightrag_vdb_{namespace}  (generic legacy format)
  3. {namespace}  (most generic, oldest format)

This ensures the migration finds the correct source collection with actual data.

Impact:
- Fixes test_backward_compat_old_workspace_naming_qdrant which creates
  a "prod_chunks" collection with 10 points
- Migration will now correctly find and migrate from workspace-specific
  legacy collections before falling back to generic collections
- Maintains backward compatibility with all legacy naming patterns

Testing:
Run: pytest tests/test_e2e_multi_instance.py::test_backward_compat_old_workspace_naming_qdrant -v

lightrag/kg/qdrant_impl.py

@@ -75,8 +75,8 @@ def _find_legacy_collection(
     This function tries multiple naming patterns to locate legacy collections
     created by older versions of LightRAG:
 
-    1. lightrag_vdb_{namespace} - Current legacy format
-    2. {workspace}_{namespace} - Old format with workspace (pre-model-isolation)
+    1. {workspace}_{namespace} - Old format with workspace (pre-model-isolation) - HIGHEST PRIORITY
+    2. lightrag_vdb_{namespace} - Current legacy format
     3. {namespace} - Old format without workspace (pre-model-isolation)
 
     Args:
@@ -88,10 +88,11 @@ def _find_legacy_collection(
         Collection name if found, None otherwise
     """
     # Try multiple naming patterns for backward compatibility
+    # More specific names (with workspace) have higher priority
     candidates = [
+        f"{workspace}_{namespace}" if workspace else None,  # Old format with workspace - most specific
         f"lightrag_vdb_{namespace}",  # New legacy format
-        f"{workspace}_{namespace}" if workspace else None,  # Old format with workspace
-        namespace,  # Old format without workspace
+        namespace,  # Old format without workspace - most generic
     ]
 
     for candidate in candidates: