fix issue wrong agent prologue for api (#2246)

### What problem does this PR solve?

#2242
### Type of change

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

agent/canvas.py

@@ -299,4 +299,7 @@ class Canvas(ABC):
                 pat = " => ".join([p.split(":")[0] for p in path[0:l]])
                 return pat + " => " + pat
 
-        return False
+        return
+
+    def get_prologue(self):
+        return self.components["begin"]["obj"]._param.prologue