Ensure GDK.output handles ASCII-8BIT data properly

What does this merge request do and why?

Previously if the output contained binary data, GDK.output.debug would error out with incompatible character encodings: UTF-8 and ASCII-8BIT. This occurred because while the prefix of the debug message was forced to UTF-8, the actual log line was not. Fix this by adding a method format_log responsible for constructing the full message and forcing the input string to UTF-8.

How to set up and validate locally

Not sure yet how to reproduce the failure, but this likely comes from when mise plugins update -y runs.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.