fix(prompt): preserve string values in dataclass unions

[ningyixi]

Loading a chat prompt whose function_call is a string template raises AttributeError: 'str' object has no attribute 'items'. PromptMessage explicitly supports str | FunctionCall | None, but deep deserialization tries the dataclass branch for every non-None value. The same error makes a valid disk-cached prompt appear to be a cache miss.

Only attempt the nested dataclass conversion when the value is a dictionary. String templates and None retain their values, while dictionary function calls still become FunctionCall objects.

Validation on the pinned Python 3.14.7/Mise environment:

• The direct prompt-loading regression and real compressed disk-cache round trip both fail before the fix; None and dictionary controls pass.
• Focused cache/serialization checks: 29 passed.
• Full test_core nox session: 1,093 passed, 106 skipped, 12 expected failures, with VCR replay only.
• Required full make fixup passes, including Ruff, formatting, Codespell and repository consistency hooks. Targeted Pylint passes with the repository's --errors-only setting.

Implementation and local validation by OpenAI Codex.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-18T04:50:35.510729Z	d829051	PR opened

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