chore: complete gapic-generator migration

[jskeet]

• Reformat with nox
• Enable generation with librarian
• Regenerate with librarian

[jskeet]

@parthea Obviously this is a huge change - but it really is just running nox format. (Librarian doesn't actually "generate" this at all.) We could potentially remove the first commit, bringing it into librarian without reformatting - but if it is okay to reformat, I think it's worth doing.

[gemini-code-assist]

Code Review

This pull request performs a comprehensive reformatting of the gapic-generator package and its integration test goldens, focusing on import organization, string quote standardization, and multi-line argument formatting. It also updates repository metadata and enables generation for the library by removing the skip_generate flag. I have no feedback to provide.

[parthea]

According to gemini, this is one of the fixes

1. Quoting Style (f-strings)
Expected (Golden): Uses single quotes for the f-string to allow literal double quotes inside:
f'Mollusc is a "{response.taxonomy}"'

Actual (Generated): Uses double quotes for the f-string and escapes the inner double quotes:
f"Mollusc is a \"{response.taxonomy}\""

2. Whitespace (Newlines)
Expected (Golden): Has an extra empty line before the # [END ...] tag.

Actual (Generated): Has no empty line before the # [END ...] tag.

I may have some fixes already in googleapis/gapic-generator-python#2499. Let me know if you want to hand this over, and what command I should run to re-generate the code after applying the fix.