AAI-236 Bugfix for datetimes in JSON

[marius-mather]

Description

AAI-236: currently getting an error "datetime is not JSON-serializable" when trying to register on Galaxy. Caused by not converting to JSON-friendly types before putting things in POST requests etc.

Changes

• Make sure we use model_dump(mode='json') before sending POST data to Auth0
• Tests for handling of types like datetime

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)

How to Test Manually (if necessary)

Run uv run pytest