Fixes #5799 Desktop entries without an `Exec` key are valid when the application is D-Bus- or systemd-activated (often with `NoDisplay=true`), so the shell can still match an icon and metadata to a GUI that isn't launched directly. `desktop-file-validate` accepts such files, but snapcraft rejected them with `missing 'Exec' key`. This makes the `Exec` reformatting conditional on the key being present instead of raising when it's absent, so the file is written through unchanged. The separate "missing Desktop Entry section" guard is untouched. The existing `test_missing_exec_entry` is converted from asserting rejection to asserting the file is written faithfully without an `Exec` line. The issue suggested delegating validation to `desktop-file-validate` rather than encoding key requirements in snapcraft. I went with this minimal in-tree fix to keep it focused and avoid a new external dependency, but I'm happy to take the delegation approach if the team prefers it. `ruff`, `ty`, and `make test-fast` (3628 passed, 0 failed) all pass. `make lint` fails locally only in its docs build (`TypeError: issubclass() arg 1 must be a class`, Sphinx on Python 3.14); I confirmed the same failure on a clean `upstream/main`, so it's pre-existing and unrelated to this change. --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/snapcraft/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [ ] I've successfully run `make lint && make test`. (Code linters and `make test-fast` pass; `make lint` docs build fails identically on clean `main`.) - [ ] I've added or updated any relevant documentation. (n/a) - [ ] In documents I changed, I added a meta description if one was missing. (n/a) - [ ] I've updated the relevant release notes. (n/a)