[Python] Add enum support when building default values for model properties when using $ref

[welshm]

Add in better processing for enums in Python when they are a $ref

• Currently, doesn't properly populate default values so when models are de/serialized, the enum value is dropped.
• Adds support for checking the $ref when getting the default value for a Schema
• Updates how enum variable names are referenced
  • Update DefaultCodegen.java with public String toEnumDefaultValue(CodegenProperty property, String value)
  • Allows checking if the property is an enum reference - and if so, handle it as a first class enum model in Python
  • Otherwise, use the inline variable value (as it does today)

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[welshm]

@wing328 @spacether @krjakbrjak I don't see Flask/Connexion as being supported (just FastAPI) but this impacts all of Python.

[wing328]

fyi @OpenAPITools/generator-core-team

[wing328]

we don't test 2.0 spec so no need to keep this sample folder and yaml config file

[welshm]

Updated - thanks!

[welshm]

Something went wrong doing the rebase - will fix

[welshm]

The code is now correct, but the commit history is a bit mangled. I can open another PR if that's a problem.

Let me know! @wing328

[wing328]

thanks for the PR. the commit history looks ok as we will squash the commits during merge.

tested python client with the new test schemas and the result is good: #18882

[wing328]

fyi. filed #18883 to add the tests in python client as well

[wing328]

@welshm when you've time, can you please PM me via Slack? thanks.