[url_launcher_linux] file resource URI support for canLaunch

[jokerttu]

Fixes canLaunch implementation for file scheme.
canLaunch incorrectly returned a false response to launchable file scheme URLs.
Fix uses same method than g_app_info_launch_default_for_uri does internally to detect default application for URI.

Fixes #93326

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[stuartmorgan-g]

Thanks for the submission! Marking as a draft per the comment above; please remove the draft designation once the tests have been reworked.

[stuartmorgan-g]

Adding @robert-ancell for second review.

[robert-ancell]

Should be good with the requested changes, thanks @jokerttu.

[stuartmorgan-g]

LGTM with a comment nit.

This will need final approval from @robert-ancell as well.

[robert-ancell]

Thanks @jokerttu!