Feat: Allow update conversation parameters and persist to database in completion (#8039)

### What problem does this PR solve?

This PR updates the completion function to allow parameter updates when
a session_id exists. It also ensures changes are saved back to the
database via API4ConversationService.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/db/services/canvas_service.py

@@ -173,6 +173,19 @@ def completion(tenant_id, agent_id, question, session_id=None, stream=True, **kw
             conv.reference = []
         conv.reference.append({"chunks": [], "doc_aggs": []})
 
+        kwargs_changed = False
+        if kwargs:
+            query = canvas.get_preset_param()
+            if query:
+                for ele in query:
+                    if ele["key"] in kwargs:
+                        if ele["value"] != kwargs[ele["key"]]:
+                            ele["value"] = kwargs[ele["key"]]
+                            kwargs_changed = True
+        if kwargs_changed:
+            conv.dsl = json.loads(str(canvas))
+            API4ConversationService.update_by_id(session_id, {"dsl": conv.dsl})
+
     final_ans = {"reference": [], "content": ""}
     if stream:
         try: