Allow failed agent turns to be forked (#2063)

* fix(chat): allow failed turns to be forked

Failed turns can end in Paseo-generated timeline items without provider
message IDs. Anchor forks to canonical timeline positions while retaining
legacy boundaries for compatibility.

Place the fork summary before the new prompt and delimit it as structured
chat history.

* fix(chat): tighten fork boundary handling

packages/app/src/agent-stream/turn-boundary.test.ts

@@ -1,6 +1,9 @@
 import { describe, expect, it } from "vitest";
 import type { StreamItem } from "@/types/stream";
-import { resolveAssistantTurnBoundaryMessageId } from "./turn-boundary";
+import {
+  resolveAssistantTurnBoundaryMessageId,
+  resolveAssistantTurnForkBoundary,
+} from "./turn-boundary";
 
 function timestamp(seed: number): Date {
   return new Date(`2026-01-01T00:00:${seed.toString().padStart(2, "0")}.000Z`);
@@ -62,3 +65,65 @@ describe("resolveAssistantTurnBoundaryMessageId", () => {
     ).toBeUndefined();
   });
 });
+
+describe("resolveAssistantTurnForkBoundary", () => {
+  it("forks a failed assistant turn from its Paseo timeline cursor without a provider message id", () => {
+    const failedTurn = {
+      ...assistantMessage("assistant-error", 2),
+      timelineCursor: { epoch: "timeline-1", seq: 42 },
+    };
+
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [userMessage("user-1", 1), failedTurn],
+        startIndex: 1,
+        supportsTimelineCursor: true,
+      }),
+    ).toEqual({
+      boundaryCursor: { epoch: "timeline-1", seq: 42 },
+    });
+  });
+
+  it("includes the provider message id with a supported timeline cursor", () => {
+    const selected = {
+      ...assistantMessage("assistant-1", 2, "msg-assistant-1"),
+      timelineCursor: { epoch: "timeline-1", seq: 42 },
+    };
+
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [selected],
+        startIndex: 0,
+        supportsTimelineCursor: true,
+      }),
+    ).toEqual({
+      boundaryCursor: { epoch: "timeline-1", seq: 42 },
+      boundaryMessageId: "msg-assistant-1",
+    });
+  });
+
+  it("falls back to the provider message id when timeline cursors are unsupported", () => {
+    const selected = {
+      ...assistantMessage("assistant-1", 2, "msg-assistant-1"),
+      timelineCursor: { epoch: "timeline-1", seq: 42 },
+    };
+
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [selected],
+        startIndex: 0,
+        supportsTimelineCursor: false,
+      }),
+    ).toEqual({ boundaryMessageId: "msg-assistant-1" });
+  });
+
+  it("does not offer an unavailable boundary", () => {
+    expect(
+      resolveAssistantTurnForkBoundary({
+        items: [assistantMessage("assistant-1", 2)],
+        startIndex: 0,
+        supportsTimelineCursor: false,
+      }),
+    ).toBeUndefined();
+  });
+});