[dart][dart-dio] Fix missing byte response cast

[kuhnroyal]

There doesn't seem to be an example for this.
Fixes #8878

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CC @agilob

[agilob]

no 'file' type used in pet clinic 3? I'm a bit surprised schema doesnt have it, I thought there was image field for pet that was file. huh

[wing328]

no 'file' type used in pet clinic 3? I'm a bit surprised schema doesnt have it, I thought there was image field for pet that was file. huh

That was for testing file upload (parameter).

For return type, looks like we've not added one for type: string, format: binary yet. I'll try to add one later this week