Serializing multiple phone numbers raises AttributeError

Problem

cv.phone accepts either one phone number or a list, but serializing a validated list crashes because Cv.serialize_phone calls .replace() on the list itself.

Reproduction

from rendercv.schema.models.cv.cv import Cv

cv = Cv.model_validate({
    "name": "Test",
    "phone": ["+14155552671", "+442071838750"],
})
cv.model_dump()

Current result:

pydantic_core.PydanticSerializationError: Error calling function `serialize_phone`:
AttributeError: 'list' object has no attribute 'replace'

This also affects the AI skill because its generated schema advertises multiple phone numbers as supported.

Expected behavior

Serialization should remove the tel: prefix from each validated phone number while preserving scalar and None behavior.

Suggested fix

Handle scalar and list values separately in the serializer and add regression coverage for both shapes.