feat(ai-chat): interrupt agent + send queued message, keeping partial output (#198)

Add a "send now" button to queued AI-chat messages: it interrupts the
running agent and immediately sends that message, while the agent's
partial output at interruption is kept in history and the next turn is
marked as a user interrupt.

Client:
- queue-helpers: pure `promoteToHead` to move a queued message to the head.
- chat-thread: `sendNow` (promote head + abort + flush-on-abort), one-shot
  `flushOnAbortRef`/`interruptNextSendRef`, `interrupted` flag in the
  request body, and the "send now" ActionIcon in the queued list.

Server:
- `interrupted` on AiChatStreamBody; pure `isInterruptResume` confirms the
  client hint against persisted history (prev assistant turn aborted/
  streaming) before honouring it.
- prompt: INTERRUPT_NOTE injected in the context section only on a
  confirmed interrupt-resume turn so the model treats the partial answer
  above as incomplete.

Tests: promoteToHead, chat-thread send-now (abort + resend + one-shot
interrupt flag + non-streaming immediate send), isInterruptResume, and
the prompt interrupt-note injection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/queue-helpers.test.ts

@@ -2,6 +2,7 @@ import { describe, it, expect } from "vitest";
 import {
   enqueueMessage,
   dequeue,
+  promoteToHead,
   removeQueuedById,
   type QueuedMessage,
 } from "./queue-helpers";
@@ -89,6 +90,52 @@ describe("removeQueuedById", () => {
   });
 });
 
+describe("promoteToHead", () => {
+  it("moves the matching id to the front, preserving the rest's order", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+      { id: "c", text: "third" },
+    ];
+    expect(promoteToHead(queue, "c")).toEqual([
+      { id: "c", text: "third" },
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ]);
+  });
+
+  it("is a no-op order-wise when the id is already the head", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ];
+    expect(promoteToHead(queue, "a")).toEqual([
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ]);
+  });
+
+  it("returns an equivalent list when the id is not present", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ];
+    expect(promoteToHead(queue, "missing")).toEqual(queue);
+  });
+
+  it("does not mutate the input queue", () => {
+    const queue: QueuedMessage[] = [
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ];
+    promoteToHead(queue, "b");
+    expect(queue).toEqual([
+      { id: "a", text: "first" },
+      { id: "b", text: "second" },
+    ]);
+  });
+});
+
 describe("FIFO order", () => {
   it("preserves order across enqueue -> dequeue", () => {
     let queue: QueuedMessage[] = [];