fix(model): update start_date type to datetime and make unit optional

[Julian-Schaefer]

Fixes langfuse/langfuse#4973

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Important

Update start_date to datetime and make unit optional in Model class.

• Model Class Updates:
  • Change start_date type from date to datetime in Model class.
  • Make unit field optional in Model class.

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[greptile-apps]

Disclaimer: Experimental PR review

PR Summary

Updates the Model class schema to support datetime precision for start dates and optional usage units, improving flexibility in model definitions.

• Changed start_date type from Optional[dt.date] to Optional[dt.datetime] in /langfuse/api/resources/commons/types/model.py for more granular timestamp support
• Modified unit field from ModelUsageUnit to Optional[ModelUsageUnit] with default None to allow model definitions without usage units
• Maintains backward compatibility through existing datetime serialization in Config class

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[greptile-apps]

logic: Making unit optional may affect cost calculations if code assumes unit is always present. Ensure all consumers handle None case.

[marcklingen]

Thanks for opening this pr

@hassiebp this needs fix in fern definition as this is autogenerated

[hassiebp]

Thanks for your contribution @Julian-Schaefer - I have used your suggested changes in #1071 and marked you as a co author. Welcome on the contributors list! https://github.com/langfuse/langfuse-python/releases/tag/v2.57.6