fix(references): repair ai-chat typecheck against current wire shape (#3685)

## Summary

Pre-existing typecheck errors in `references/ai-chat` against the
current SDK shape. Unblocks `pnpm exec tsc --noEmit` in the reference
project.

## What changed

Three categories of fixes inside `references/ai-chat`. No SDK changes.

### 1. `payload.messages` → `payload.message`

The wire payload is now delta-only — one new message per trigger,
optional. Old code in two raw-task files reads `payload.messages`
(plural array) which no longer exists.

```ts
// before
const messages = await conversation.addIncoming(currentPayload.messages, ...);

// after
const messages = await conversation.addIncoming(
  currentPayload.message ? [currentPayload.message] : [],
  ...
);
```

Same fix to the `chat.messages.on` handler, reading `msg.message`
(singular) instead of `msg.messages[length - 1]`.

### 2. `clientData` non-null assertion in `cf-trust-test`

`ChatTurnContext.clientData` is typed as `?: TClientData` on
`onTurnStart` / `run` event objects even when the agent declares a
`clientDataSchema`. The runtime validates against the schema before the
hook fires, so it's structurally non-null — but TypeScript can't know
that. Non-null assert for now.

Follow-up worth filing: narrow `ChatTurnContext.clientData` to
non-optional when the agent has a `clientDataSchema`. Same friction the
docs friction-test subagent flagged.

### 3. `stress-emit.parseConfig` retyped against `ModelMessage[]`

The `run` callback hands `messages: ModelMessage[]`, not `UIMessage[]`.
Update `parseConfig` to accept `ModelMessage[]` and pull text from
`content` (string or array-of-parts).

## Test plan

- [x] `pnpm exec tsc --noEmit` in `references/ai-chat` passes (was 8
errors, now 0)

references/ai-chat/src/trigger/chat-client-test.ts

@@ -272,7 +272,7 @@ export const orchestratorAgent = chat
         stop.reset();
 
         const messages = await conversation.addIncoming(
-          currentPayload.messages,
+          currentPayload.message ? [currentPayload.message] : [],
           currentPayload.trigger,
           turn
         );

references/ai-chat/src/trigger/chat.ts

@@ -659,7 +659,7 @@ export const aiChatRaw = chat.customAgent({
       stop.reset();
 
       const messages = await conversation.addIncoming(
-        currentPayload.messages,
+        currentPayload.message ? [currentPayload.message] : [],
         currentPayload.trigger,
         turn
       );
@@ -678,8 +678,7 @@ export const aiChatRaw = chat.customAgent({
       const combinedSignal = AbortSignal.any([runSignal, stop.signal]);
 
       const steeringSub = chat.messages.on(async (msg) => {
-        const lastMsg = msg.messages?.[msg.messages.length - 1];
-        if (lastMsg) await conversation.steerAsync(lastMsg);
+        if (msg.message) await conversation.steerAsync(msg.message);
       });
 
       const result = streamText({
@@ -1049,10 +1048,10 @@ export const cfTrustTestAgent = chat
     id: "cf-trust-test",
     idleTimeoutInSeconds: 60,
     onTurnStart: async ({ turn, clientData }) => {
-      logger.info("cf-trust-test turn", { turn, cf: clientData.__cf, userId: clientData.userId });
+      logger.info("cf-trust-test turn", { turn, cf: clientData!.__cf, userId: clientData!.userId });
     },
     run: async ({ messages, clientData, signal }) => {
-      const cf = clientData.__cf;
+      const cf = clientData!.__cf;
       return streamText({
         model: openai("gpt-4o-mini"),
         system:

references/ai-chat/src/trigger/stress-emit.ts

@@ -10,7 +10,7 @@
 // Defaults: 1000 chunks × 10 chars, single message.
 
 import { chat } from "@trigger.dev/sdk/ai";
-import { type UIMessage, simulateReadableStream, streamText } from "ai";
+import { type ModelMessage, simulateReadableStream, streamText } from "ai";
 import { MockLanguageModelV3 } from "ai/test";
 import type { LanguageModelV3StreamPart } from "@ai-sdk/provider";
 
@@ -20,10 +20,16 @@ type StressConfig = {
   manyMessages: boolean;
 };
 
-function parseConfig(messages: UIMessage[]): StressConfig {
+function parseConfig(messages: ModelMessage[]): StressConfig {
   const lastUser = [...messages].reverse().find((m) => m.role === "user");
-  const text =
-    lastUser?.parts?.[0]?.type === "text" ? lastUser.parts[0].text.trim() : "";
+  const content = lastUser?.content;
+  let text = "";
+  if (typeof content === "string") {
+    text = content.trim();
+  } else if (Array.isArray(content)) {
+    const textPart = content.find((p) => p.type === "text");
+    text = textPart && "text" in textPart ? textPart.text.trim() : "";
+  }
   const parts = text.split(/\s+/);
   const chunkCount = Number(parts[0]);
   const chunkSize = Number(parts[1]);