Writing any datetime field raises TypeError: model_to_payload dumps in python mode

[baraline]

Symptom

Writing any datetime field raises TypeError before the request leaves the process.

from glpi_python_client.models.api_schema.assistance.timeline import PostTicketTask

await client.create_ticket_task(1, PostTicketTask(planned_begin=datetime(2024, 1, 1, 12, 0)))
# TypeError: Object of type datetime is not JSON serializable

Root cause

model_to_payload (_async/clients/commons/_payloads.py:26) calls model.model_dump(...) in python
mode, so datetime fields stay as datetime objects in the body dict. _execute_request
(_async/clients/commons/_transport.py:233) hands that dict to httpx as json=, whose encoder is
json.dumps.

Reproduced directly:

>>> httpx.Request("POST", "http://x/", json={"planned_begin": datetime.datetime(2024, 1, 1)})
TypeError: Object of type datetime is not JSON serializable

Naive and aware values fail identically.

Why the suite is green

TransportRecorder stubs at client._session.request, i.e. above the httpx JSON encoder, so no unit
test ever encodes a payload. The bug is only reachable against a real socket.

Affected surface

Every model with a declared, writable datetime field: PostTicketTask/PatchTicketTask
(planned_begin, planned_end, date), PostFollowup, PostSolution, PostUser/PatchUser
(begin_date, end_date, substitution_*), and the KB post models.

Fix direction

model_dump(mode="json", exclude_none=True, exclude={"extra_payload"}).

This decides the outbound wire format, so it must land before the aware-datetime work: once inbound
values are aware, mode="json" starts emitting +01:00/Z offsets into GLPI's MySQL DATETIME columns,
which is a separate decision.

Needs a test that asserts the payload is JSON-encodable, not merely that the right dict was produced.