Fix broken Fuseki when using a context path in the URL

### Version

main e9c3c3058a056438fd3de32e5cfdf658e4a6fb20

### What happened?

We fixed this in #2392 a few days ago, but apparently something broke that again (see same issue linked).

https://github.com/apache/jena/issues/2392#issuecomment-2059409976 reported by OP @peutch

- [ ] fix it again
- [ ] write a test to avoid future regressions

### Relevant output and stacktrace

```shell
Quoting the comment linked above.

> using the main branch is breaking again something (http://localhost:3030/fuseki does not serve index.html and forcing http://localhost:3030/fuseki/index.html goes wrong).
> using e1e9bf4 and just cherry picking 2f3a293 is working as expected.
> Probably one commit in between breaks something. One of the dep updates in 3a09ec5? Didn't tried to bisect.
```

### Are you interested in making a pull request?

Maybe

Start by comparing the main revision e9c3c3058a056438fd3de32e5cfdf658e4a6fb20 with e1e9bf4 and the working cherry-pick 2f3a293, focusing on the changes around dependency update 3a09ec5 and issue #2392. Reproduce the failure at /fuseki and /fuseki/index.html, then identify the relevant Fuseki entry point and add a regression test. Done means both URLs serve the expected index page again.

Written by the indexing model from the issue text.