fix(api): allow canvas_type in agent create and update APIs (#15201)

### What problem does this PR solve?

Creating or updating an agent via `POST /api/v1/agents` and `PUT
/api/v1/agents/{agent_id}` did not persist `canvas_type` because the
handler `req` dict never assigned the field before
`UserCanvasService.save` / `update_by_id`.


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

Co-authored-by: Cursor <cursoragent@cursor.com>

api/apps/restful_apis/agent_api.py

@@ -616,6 +616,7 @@ async def update_agent_tags(tenant_id, canvas_id):
 @add_tenant_id_to_kwargs
 async def create_agent(tenant_id):
     req = {k: v for k, v in (await get_request_json()).items() if v is not None}
+    req["canvas_type"] = req.get("canvas_type","")
     req["user_id"] = tenant_id
     req["canvas_category"] = req.get("canvas_category") or CanvasCategory.Agent
     req["release"] = bool(req.get("release", ""))
@@ -872,6 +873,7 @@ def delete_agent(agent_id, tenant_id):
 @_require_canvas_access_async
 async def update_agent(agent_id, tenant_id):
     req = {k: v for k, v in (await get_request_json()).items() if v is not None}
+    req["canvas_type"] = req.get("canvas_type","")
     req["release"] = bool(req.get("release", ""))
 
     if req.get("dsl") is not None: