Fix 500 error when creating contestant without birth_date

[Copilot]

Contestant.serialize_birth_date unconditionally called .isoformat() on the value, crashing with AttributeError: 'NoneType' object has no attribute 'isoformat' when the optional birth_date field was omitted.

Changes

• app/models/contestant_model.py: Update serialize_birth_date to accept date | None and return None when no date is provided — consistent with how serialize_email already handles its optional field:

# Before
@field_serializer("birth_date")
def serialize_birth_date(self, dt: date) -> str:
    return dt.isoformat()

# After
@field_serializer("birth_date")
def serialize_birth_date(self, dt: date | None) -> str | None:
    return dt.isoformat() if dt else None

• tests/integration/test_contestants.py: Add test_create_contestant_without_birth_date covering the POST /events/{id}/contestants path with birth_date=None.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• releases.astral.sh
  • Triggering command: /home/REDACTED/.local/bin/uv /home/REDACTED/.local/bin/uv run pytest tests/integration/test_contestants.py::test_create_contestant_without_birth_date tests/integration/test_contestants.py::test_create_contestant_good_case -v (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)