fix: allow for using datetime in list, dict and pydantic model

[mattias-lundell]

No description provided.

[agentobot]

🚀 Summary

This PR introduces ISO-8601 serialization for datetime objects in prepare_input and switches Pydantic handling to model_dump(mode="json"). The accompanying tests exercise basic types, nested collections, and datetimes. Version bumped to 0.31.4 and CI config remains unchanged.

✅ What Looks Good

Area	Notes
Correctness	Datetime → ISO string conversion works and is unit-tested.
Test Coverage	78 new LOC cover positive & mixed-type cases; tests read well.
Style & Readability	Implementation is concise; no lint issues observed.
Backwards Compatibility	Existing behaviour for str/int/bool/None unaffected.

💡 Suggestions / Non-blocking

1. Timezone awareness – Consider adding a test for timezone-aware datetimes (datetime.now(timezone.utc)) to ensure correct offset preservation.
2. Redundant logic – DateTimeEncoder.default() duplicates the new isinstance(x, datetime) branch in prepare_input. You could rely on one path to avoid divergence.
3. Changelog / Docs – Add a note in CHANGELOG or README about datetime support so downstream users discover it easily.
4. Microseconds – If API consumers don’t need microsecond precision, consider stripping them to reduce payload noise.

Overall these are polish items and do not block merging.

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Reviewed by agentobot[bot] – bringing research-backed, high-impact feedback