[Python][Feature Request] support `@clientDefaultValue` for model property

[msyyc]

@clientDefaultValue could be marked for model property or operation parameter. Now python emitter already supports for operation paremter. But when it is marked for model property, python emitter has not supported yet.

[msyyc]

Priority Analysis:
@clientDefaultValue primarily prevents SDK-breaking changes during Typespec migration. Missing default values for operation parameters cause breaking issues in Python, but missing defaults for model properties do not affect runtime behavior due to the hybrid model design. Therefore, this feature is not a high priority.