Broken type for useServerFn inside useMutation

Which project does this relate to?

Router

Describe the bug

Hi, I think the type is broken for the useServerFn.
I've reproduced and noticed that if I need to use the onSuccess callback I loose the types

Your Example Website or App

https://github.com/Christopher96u/ts-issue-demo

Steps to Reproduce the Bug or Issue

1. Go to the index/home route

Expected behavior

I expect as user, to have the onSuccess types back or improve the docs if this is the right behaviour

Screenshots or Videos

If I comment out the onSuccess callback there is no TS issues

If I leave the onSuccess callback there is a TS issue

If we use the useServerFn hook at top level (not inside the useMutation hook, we get the types back

Platform

• Router / Start Version: [e.g. 1.121.0]
• OS: [e.g. macOS, Windows, Linux]
• Browser: [e.g. Chrome, Safari, Firefox]
• Browser Version: [e.g. 91.1]
• Bundler: [e.g. vite]
• Bundler Version: [e.g. 7.0.0]

Additional context

No response

Start with the index/home route in the linked ts-issue-demo reproduction and inspect how useServerFn is passed into useMutation, especially the onSuccess callback. Compare that with using useServerFn at the top level. Done means the callback types are preserved, or the documentation clearly explains the intended behavior.