[Core] Add Serializable protocol

[pvaneck]

This is a protocol that custom models can implement to allow it to work with the SDK serialization/deserialization mechanisms.

Ref: https://gist.github.com/pvaneck/3f7266c07828d0fb3bd15e8cfbc618d5#2-protocol-based-approach

Open questions:

• Are these method names too general? Could name collisions be a possibility?

[annatisch]

Are we expecting people to implement any internal recursion themselves? Or do we do that for them?

I was wondering if instead of using Dict[str, Any], whether we want to lock the value types down to the types that we know our JSON encoder supports....
Not sure how feasible and/or useful that would be....

[pvaneck]

The SdkJSONEncoder does automatically handle nested custom types and will call the nested type to_dict methods internally. E.g.: if someone implements to_dict like:

Class FooModel:
     
    foo: str
    bar: BarModel

    ...

    def to_dict(self) -> Dict[str, Any]:
        return {"foo": self.foo, "bar": self.bar}

bar.to_dict() will be called internally during serialization.

For as_dict(), we'd need some slight adjustments in the code. This would probably just be wrapping the output of to_dict with another _serialize call in the model_base.py file

Regarding the typing, I think keeping it simpler with Any would be more flexible, especially if we are allowing output like the above with arbitrary types being values. The Union would be pretty large, and it's somewhat of a maintenence burden if our JSON encoder adds support for another type, as we'd have to update the Union.

[Copilot]

Pull Request Overview

This PR introduces a new Serializable protocol to Azure Core that allows custom models to integrate with the SDK's serialization and deserialization mechanisms. The protocol defines standard to_dict() and from_dict() methods that custom types can implement to control their JSON representation.

• Adds a runtime-checkable Serializable protocol with to_dict and from_dict methods
• Updates the JSON encoder to handle objects implementing the Serializable protocol
• Extends deserialization logic to work with custom serializable models
• Provides comprehensive test coverage for both standalone and embedded custom models

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
azure/core/serialization.py	Defines the new Serializable protocol with documentation and type annotations
modeltypes/_utils/model_base.py	Updates SdkJSONEncoder and deserialization logic to handle Serializable objects
tests/test_serialization.py	Adds comprehensive test suite for the new protocol functionality

[Copilot]

The Serializable protocol should be added to __all__ to make it part of the public API since it's intended for external use by custom models.

Suggested change

	__all__ = ["NULL", "AzureJSONEncoder", "is_generated_model", "as_attribute_dict", "attribute_list"]
	__all__ = ["NULL", "AzureJSONEncoder", "is_generated_model", "as_attribute_dict", "attribute_list", "Serializable"]

[Copilot]

This line duplicates the dictionary definition from line 1055-1059. Remove this duplicate assignment.

Suggested change

json_dict = {"foo": "foo", "bar": 42, "baz": 3.14}

[Copilot]

This check is incorrect. annotation is a type/class, not an instance. Use issubclass(annotation, Serializable) instead of isinstance(annotation, Serializable).

Suggested change

	if isinstance(annotation, Serializable):
	return annotation.from_dict # type: ignore
	try:
	if issubclass(annotation, Serializable):
	return annotation.from_dict # type: ignore
	except TypeError:
	pass