Fix Polars temporal column stats JSON serialization for tracker

[dgvj-work]

Make Polars temporal column stats JSON-safe and cover the #1127 Datetime regression.

Changes

• Extend datetime_column_stats conversion so Date/Datetime/Time/Duration values are JSON-serializable for the UI
• Keep emitting base_data_type="datetime" (no new stats type)
• Add Datetime Series regression test matching Hamilton tracker gives an error with datetime columns in polars dataframes #1127, plus Time/Duration coverage

How I tested this

• pytest ui/sdk/tests/tracking/test_polars_col_stats.py ui/sdk/tests/tracking/test_polars_stats.py (23 passed)
• pre-commit run ruff-check / ruff-format on the touched files (passed)

Notes

• On main, temporal columns already avoid the original std() crash by routing through datetime_column_stats with std=0.0, but Hamilton tracker gives an error with datetime columns in polars dataframes #1127 was still open: there was no regression test pinning that Datetime path, and Time/Duration values on the same path were still not JSON-serializable for the tracker payload. This PR closes that loop.

Closes #1127

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

Drafted with AI assistance (Cursor); I verified the serialization behavior against polars' temporal dtypes manually and reviewed all test assertions.