fix: release the openGauss connection when a read returns no rows (#30144)

An openGauss query or get that finds nothing returns before the rollback that ends its read-only transaction, so the session keeps the connection it checked out of the pool. Retrieval fans out over worker threads and the session is thread-local, so every thread that runs an empty read holds a connection for the lifetime of that thread, and vector search eventually fails with a pool checkout timeout that a restart is the only way out of. Empty reads are routine: an empty knowledge base, a file whose chunks were deleted, or a metadata filter that matches nothing all produce one.

Rolling back before the two early returns puts the connection back. Measured by driving OpenGaussClient itself with a pool of 5: three empty query reads left 3 connections checked out and 0 free before the change, and 0 checked out with 3 free after, same for get. search is already correct, it has no early return.

The pgvector client had the same defect, fixed separately in #30142.

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

@@ -311,6 +311,7 @@ class OpenGaussClient(VectorDBBase):
             results = query.all()
 
             if not results:
+                self.session.rollback()
                 return None
 
             ids = [[result.id for result in results]]
@@ -333,6 +334,7 @@ class OpenGaussClient(VectorDBBase):
             results = query.all()
 
             if not results:
+                self.session.rollback()
                 return None
 
             ids = [[result.id for result in results]]