fix: allow zero message history window and clear history for new sessions (#15897) (#15902)

### What problem does this PR solve?

Two bugs in the Agent Categorize component:
1. The backend rejected `message_history_window_size = 0` while frontend
allowed it, causing API errors.
2. When calling the agent API without a `session_id`, a new session was
created but retained history from previous conversations.

### Type of change

- [x] Bug Fix

### How has this been tested?

- Issue 1: `CategorizeParam().check()` now accepts `0` and rejects
negative values.
- Issue 2: `canvas.clear_history()` is called for new sessions (no
`session_id`), ensuring fresh conversation state. Verified via UI and
API that a second call without `session_id` does not remember the first
conversation.

### Related Issue

Closes #15897

Co-authored-by: RAGFlow Dev <dev@ragflow.local>
Co-authored-by: Wang Qi <wangq8@outlook.com>

agent/canvas.py

@@ -329,6 +329,11 @@ class Canvas(Graph):
         self.dsl["memory"] = self.memory
         return super().__str__()
 
+    def clear_history(self):
+        self.history = []
+        if isinstance(self.globals.get("sys.history"), list):
+            self.globals["sys.history"] = []
+
     def reset(self, mem=False):
         super().reset()
         if not mem:

agent/component/categorize.py

@@ -40,7 +40,8 @@ class CategorizeParam(LLMParam):
         self.update_prompt()
 
     def check(self):
-        self.check_positive_integer(self.message_history_window_size, "[Categorize] Message window size > 0")
+        if not isinstance(self.message_history_window_size, int) or self.message_history_window_size < 0:
+            raise ValueError("[Categorize] Message window size cannot be negative")
         self.check_empty(self.category_description, "[Categorize] Category examples")
         for k, v in self.category_description.items():
             if not k:

api/apps/restful_apis/agent_api.py

@@ -1447,6 +1447,7 @@ async def agent_chat_completion(tenant_id, agent_id=None):
             from agent.canvas import Canvas
 
             canvas = Canvas(dsl_str, str(tenant_id), canvas_id=agent_id, custom_header=custom_header)
+            canvas.clear_history()
         except Exception as exc:
             return server_error_response(exc)
         turn_id = get_uuid()