closes#10451
### Tests for Go changes
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
- [ ] `make pr-go` before pushing
### Release notes
- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
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## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Bug fixes
- [PR](https://codeberg.org/forgejo/forgejo/pulls/12643): <!--number 12643 --><!--line 0 --><!--description cmV0dXJuIEFQSSBVUkwgaW4gdGhlIGB1cmxgIGZpZWxkIGZvciBwdWxsIHJlcXVlc3RzIHVzaW5nIHRoZSBBUEk=-->return API URL in the `url` field for pull requests using the API<!--description-->
<!--end release-notes-assistant-->
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12643
Reviewed-by: Robert Wolff <mahlzahn@posteo.de>
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
