fix(FN-XXX): unify codex auth and chat fallback

packages/engine/src/fallback-model-observer.ts

@@ -0,0 +1,52 @@
+import { notifyFallbackUsed } from "./notifier.js";
+import type { FallbackModelUsedPayload } from "./pi.js";
+
+type FallbackLogStore = {
+  logEntry?(taskId: string, action: string): Promise<unknown>;
+  appendAgentLog?(
+    taskId: string,
+    text: string,
+    type: "text" | "thinking" | "tool" | "tool_result" | "tool_error",
+    detail?: string,
+    agent?: string,
+  ): Promise<unknown>;
+};
+
+type FallbackModelObserverOptions = {
+  agent: string;
+  label: string;
+  store?: FallbackLogStore;
+  taskId?: string;
+  taskTitle?: string;
+};
+
+function buildFallbackLogMessage(
+  label: string,
+  payload: FallbackModelUsedPayload,
+): string {
+  return `[fallback] ${label} switched from ${payload.primaryModel} to ${payload.fallbackModel} (${payload.triggerPoint})`;
+}
+
+export function createFallbackModelObserver(options: FallbackModelObserverOptions) {
+  return async (payload: FallbackModelUsedPayload): Promise<void> => {
+    const taskId = options.taskId ?? payload.taskId;
+    const taskTitle = options.taskTitle ?? payload.taskTitle;
+    const message = buildFallbackLogMessage(options.label, payload);
+
+    if (taskId && options.store?.logEntry) {
+      await options.store.logEntry(taskId, message).catch(() => undefined);
+    }
+    if (taskId && options.store?.appendAgentLog) {
+      await options.store.appendAgentLog(taskId, message, "text", undefined, options.agent).catch(() => undefined);
+    }
+
+    await notifyFallbackUsed({
+      primaryModel: payload.primaryModel,
+      fallbackModel: payload.fallbackModel,
+      triggerPoint: payload.triggerPoint,
+      taskId,
+      taskTitle,
+      timestamp: payload.timestamp,
+    });
+  };
+}