feat(langgraph): replace toLangGraphEventStream with encoding argument

[dqbd]

Supersedes toLangGraphEventStream and toLangGraphEventStreamResponse. Similar to encoding option in streamEvents, but emits LGP-style events, that can be consumed via custom transport in useStream (see #1662).

Example usage:

const app = new Hono();

app.post("/api/stream", async (c) => {
  const { input } = z.object({ input: schema }).parse(await c.req.json());

  const stream = await graph.stream(input, {
    encoding: "text/event-stream",
    streamMode: ["values", "messages", "updates"],
  });

  return new Response(stream, {
    headers: { "Content-Type": "text/event-stream" },
  });
});

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