Fix：The OpenAI-Compatible Agent API returns an incorrect message (#8177)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/8175

### Type of change

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

agent/canvas.py

@@ -169,6 +169,7 @@ class Canvas:
     def run(self, running_hint_text = "is running...🕞", **kwargs):
         if not running_hint_text or not isinstance(running_hint_text, str):
             running_hint_text = "is running...🕞"
+        bypass_begin = bool(kwargs.get("bypass_begin", False))
 
         if self.answer:
             cpn_id = self.answer[0]
@@ -188,6 +189,12 @@ class Canvas:
         if not self.path:
             self.components["begin"]["obj"].run(self.history, **kwargs)
             self.path.append(["begin"])
+            if bypass_begin:
+                cpn = self.get_component("begin")
+                downstream = cpn["downstream"]
+                self.path.append(downstream)
+
+
 
         self.path.append([])