fix(agent): 修复流式推理与工具参数重试

backend/internal/app/cloud_agent_json.go

@@ -9,6 +9,16 @@ import (
 
 var errCloudAgentJSONSingleObject = errors.New("参数必须是单个 JSON 对象")
 
+// Only syntax/schema errors may be repaired by the model; authorization and
+// unsupported mutations still fail admission before any write or approval.
+type cloudAgentArgumentError struct{ error }
+
+func (e *cloudAgentArgumentError) Unwrap() error { return e.error }
+
+func canvasArgumentError() error {
+	return &cloudAgentArgumentError{BadAuthRequest("画布工具参数无效：仅允许一个 JSON 对象；顶层只含 snapshotHash 和 ops，snapshotHash 不得放入 ops。请按工具 schema 修正后重试")}
+}
+
 // decodeCloudAgentJSONObject is used for model tool arguments. Tool arguments
 // are an untrusted protocol boundary: reject non-objects, unknown fields and
 // trailing JSON instead of silently accepting an ambiguous payload.