CloudEvent serializes invalid extension attribute names without validation

[jack-burridge-eit]

CloudEvent serializes invalid extension attribute names without validation

Repro:

from cloudevents.v1.http import CloudEvent
from cloudevents.v1.conversion import to_structured

event = CloudEvent(
  {
      "type": "example.event",
      "source": "/test",
      "example-extension": "123",
  },
  {"ok": True},
)

_, body = to_structured(event)
print(body)

Expected: construction or serialization rejects example-extension, because CloudEvents attribute names must only contain lowercase ASCII letters and digits.

Actual: the SDK serializes it successfully, producing an invalid CloudEvent. This can fail at runtime with stricter receivers such as Knative Eventing, which rejects the request with 400 Bad Request.