docs(sdk): correct the createSession example to use chat.customAgent

The docstring showed a plain task(), but createSession resolves its
stop signal and message channels through the session handle that only
the chat.customAgent (or chat.agent) wrapper binds at run boot. A plain
task() would throw "session handle is not initialized" on the first
turn. Aligns the example with the reference project and the docs.

Author: ericallam

packages/trigger-sdk/src/v3/ai.ts

@@ -9003,14 +9003,18 @@ export type ChatTurn = {
  * signaling, and idle/suspend between turns. You control: initialization,
  * model/tool selection, persistence, and any custom per-turn logic.
  *
+ * Call from inside a `chat.customAgent()` run — the wrapper binds the
+ * backing Session that the iterator's stop signal and message channels
+ * resolve to. (A plain `task()` does not bind it, so `createSession`
+ * would throw "session handle is not initialized".)
+ *
  * @example
  * ```ts
- * import { task } from "@trigger.dev/sdk";
  * import { chat, type ChatTaskWirePayload } from "@trigger.dev/sdk/ai";
  * import { streamText } from "ai";
  * import { openai } from "@ai-sdk/openai";
  *
- * export const myChat = task({
+ * export const myChat = chat.customAgent({
  *   id: "my-chat",
  *   run: async (payload: ChatTaskWirePayload, { signal }) => {
  *     const session = chat.createSession(payload, { signal });