Simplify encoding kw_only args and test init=False args.

[benfei]

This change replaces the helper function for determining whether to use kw_only with a simpler condition (simpler to follow, easier for type checking).
It also adds test coverage for the init=False case.

addresses #36978
coordinated with @assaf127

[gemini-code-assist]

Summary of Changes

Hello @benfei, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the logic for identifying kw_only arguments in dataclasses within the coder implementation, replacing a helper function with a more direct and type-checker-friendly condition. Concurrently, it expands the test suite to include comprehensive coverage for dataclasses that utilize init=False fields alongside kw_only arguments, ensuring robust and correct serialization behavior.

Highlights

• Simplified kw_only argument encoding: The helper function dataclass_uses_kw_only has been removed and its logic inlined directly into the encoding process, resulting in simpler and more type-checker-friendly code.
• Enhanced test coverage for init=False fields: New test cases have been added to specifically cover the correct encoding of dataclasses that include init=False fields alongside kw_only arguments, ensuring robust serialization behavior.

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