fix: chroma has_collection always returns False (name vs Collection) (#25780)

has_collection did `collection_name in self.client.list_collections()`,
but chromadb's list_collections() returns Collection objects (1.x), not
name strings — so the membership test is always False, even when the
collection exists. Compare against the collection names instead (with a
hasattr guard tolerating versions that yield plain names).

Found via the dependency-contract test suite (unit/deps/test_chromadb.py).

backend/open_webui/retrieval/vector/dbs/chroma.py

@@ -57,7 +57,11 @@ class ChromaClient(VectorDBBase):
 
     def has_collection(self, collection_name: str) -> bool:
         # Check if the collection exists based on the collection name.
-        collection_names = self.client.list_collections()
+        # chromadb's list_collections() returns Collection objects (1.x), so a
+        # bare `name in collections` membership test is always False — compare
+        # against the names. (hasattr guard tolerates versions that yield names.)
+        collections = self.client.list_collections()
+        collection_names = [c.name if hasattr(c, 'name') else c for c in collections]
         return collection_name in collection_names
 
     def delete_collection(self, collection_name: str):