Replace freezegun with time-machine

[r4victor]

Fixes #4357. freezegun replaces datetime.datetime with FakeDatetime while frozen, which pydantic 2.14 can't build schemas for. time-machine patches the clock functions at the C level and keeps the real datetime class.

• Replace freeze_time(X) with time_machine.travel(X, tick=False), and class-level @freeze_time with the @pytest.mark.time_machine marker.
• Use get_current_datetime() instead of naive datetime.now() for gateway ChatModel.created_at, so the registry test doesn't depend on the local timezone.
• Remove the FakeDatetime route schema warm-up from the server tests conftest.py.