Set fields when defer_build is set on Pydantic dataclasses

[Viicos]

This aligns with Pydantic models, where only the schema generation is deferred.

Also fix some comments.

Fixes #10953.

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• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
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[codspeed-hq]

CodSpeed Performance Report

Merging #10984 will not alter performance

_{Comparing dataclass-defer-build-fields (48ff78b) with main (6f31f8f)}

Summary

✅ 46 untouched benchmarks

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_dataclasses.py
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[sydney-runkle]

Great, thanks for the simple fix!