feat(tanstackstart-react): Trace server functions #18513

This PR adds tracing for tss server functions. To achieve this I added a new withSentry wrapper that can be used to instrument the tss server entry point:

import { withSentry } from '@sentry/tanstackstart-react';

import handler, { createServerEntry } from '@tanstack/react-start/server-entry';

const requestHandler = withSentry({
  fetch(request: Request) {
    return handler.fetch(request);
  },
});

export default createServerEntry(requestHandler);

With this we get spans for server functions executed via fetch calls to the server. A limitation of this approach is that out-of-the-box this will only start a single span for the initial request made to the server. So for instance if a server function calls another server function, we will still only get a single span for the outer server function and users would need to wrap the inner call manually.

Screenshot from my sample app with the current state:

Tests added:

• Basic transaction test to verify that we get spans if a server function is executed.
• Another transaction test documenting that users need to manually wrap "nested" server functions.