Allow vanilla (de)serialization of discriminated unions

[DiamondJoseph]

Fixes #130, #131

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.25%. Comparing base (f49320c) to head (688265e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
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- Coverage   96.31%   96.25%   -0.06%     
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  Files           9        9              
  Lines         949      935      -14     
==========================================
- Hits          914      900      -14     
  Misses         35       35              

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[DiamondJoseph]

Current handling requires that subclasses of type annotated with discriminated_union_of_subclasses are Pydantic dataclasses, which I will add to the docstring: this way their schema is generated after Spec has had it schema updated.

Types that want to have their schema updated when a new instance of a class decorated with discriminated_union_of_subclasses must also either be Pydantic dataclasses or BaseModels.

The FastAPI schema for the routes into the service does not update when a new subclass is created, nor do any types that refer to types that refer to a discriminated union.

If Pydantic exposed some way of propagating that a schema that is used in the schema of a model has updated, so the schema of the model must also update, then _TaggedUnion could remove all handling of references and just rebuild_dataclass(self._baseclass)

[DiamondJoseph]

I've made an issue for that last part, but I don't think it will gain much traction as I think creating runtime models often goes against the purpose of validation

pydantic/pydantic#10091

[callumforrester]

LGTM, minor points only, thanks for this!

[ZohebShaikh]

Suggested change

	cls: The superclass of the union, Expression in the above example
	super_cls: The superclass of the union, Expression in the above example

[ZohebShaikh]

LGTM, Thank you Joseph