Start: page route returns 500 when Accept excludes HTML

Which project does this relate to?

Start

Describe the bug

createStartHandler refuses to render a page route when the request's Accept
header does not allow HTML, and reports that refusal as 500:

https://github.com/TanStack/router/blob/179d9b9b5eec50dc7833b4e2ef91b735d34bbc07/packages/start-server-core/src/createStartHandler.ts#L558-L561

A failed content negotiation is a client-side condition, so this should be
406 Not Acceptable. 500 says the server malfunctioned, and nothing did.

Complete minimal reproducer

reproduction doesn't need any custom code

Steps to Reproduce the Bug

Against any Start app, on any page (non-server) route:

  curl -i -H 'Accept: application/json' https://example.com/
  curl -i -H 'Accept: image/webp'       https://example.com/

Both return 500 {"error":"Only HTML requests are supported here"}. Accept: /, a browser Accept, and an absent header all return 200.

Expected behavior

Should return 406 Not Acceptable instead of 500 Server Error.

Screenshots or Videos

No response

Platform

• @tanstack/react-start 1.168.32
• @tanstack/start-server-core 1.169.17 (current latest)

Additional context

No response

Start at packages/start-server-core/src/createStartHandler.ts around lines 558-561, then reproduce the issue with the provided curl commands and Accept headers. Done means non-HTML Accept values return 406 Not Acceptable, while /, browser Accept headers, and an absent header continue returning 200.