fix(generator): write using utf-8

[AmaseCocoa]

Change Summary

Changed to explicitly use utf-8 in packaged_schema.write_text to avoid UnicodeEncodeError that cp932 may cause in Japanese Windows environment.

Unit Test is currently under construction :)

Checklist

• Unit tests for the changes exist
• Tests pass without significant drop in coverage
• Documentation reflects changes where applicable
• Test snapshots have been updated if applicable

Agreement

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[AmaseCocoa]

For some reason it seems to contain commit for pull request #1002 :( I forgot that I committed to main by mistake, it seems it has already been Revert.