doc(asgi): Change custom route example to PlainTextResponse

[mcw0933]

What

Updates the doc examples of custom routes to use PlainTextResponse instead of JSONResponse

Why

streamable_http expects the client to connect with a header for text/event-stream - so if you actually use the old example, you'll fail when trying to return JSON from Starlette/FastAPI with this error message:

{
  "jsonrpc": "2.0",
  "id": "server-error",
  "error": {
    "code": -32600,
    "message": "Not Acceptable: Client must accept text/event-stream"
  }
}

which makes it seem like a client issue, but it's actually just the response body not being plain text.

[jlowin]

Good catch, thank you!

[mcw0933]

No prob! Love your documentation! ❤️ We're out here rolling our MCP server now and FastMCP has been a godsend 😁

Any chance you will be at PyCon this weekend?

[jlowin]

Unfortunately not this one but maybe we’ll meet another time!