fix: allow desktop files without an Exec key

[WaiHlyanMinThein17]

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.

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