Fix: add prologue to api. (#9322)

### What problem does this PR solve?


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/sdk/session.py

@@ -862,5 +862,6 @@ def begin_inputs(tenant_id, agent_id):
             "title": cvs.title,
             "avatar": cvs.avatar,
             "inputs": canvas.get_component_input_form("begin"),
+            "prologue": canvas.get_prologue()
         }
     )