[scala-http4s] Fix name mapping of discriminator property

[MichaelKutzner]

Fixes #23792
This updates model.mustache for the scala-http4s generator to properly use propertyBaseName instead of propertyName when using nameMappings. Generated Decoders will no longer use the mapped names for lookup, but the actual property. For Encoders the opposite holds true and the generator will now set the expected property.

Notice that there are four cases handled, which should all be updated with this fix:

• No mapping of the discriminator value, no nested oneOfs
• Mapping of the discriminator value, no nested oneOfs
• No mapping of the discriminator value, with nested oneOfs
• Mapping of the discriminator value, with nested oneOfs

For a better understanding, I have updated the example openapi.yaml file from the issue, so each of these cases can be tested, if the corresponding lines are uncommented:
https://github.com/MichaelKutzner/openapi-discriminator-example/blob/50d8b38a5813107a98a699c60a18711b019d0036/openapi.yaml#L50-L56

For verification, check that the generated code uses type instead of `type`, as explained in the issue.

As I could change all cases without breaking any tests, I think, that it might be good to make some improvements there as well. But I don't know where to start.

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Summary by cubic

Fixes discriminator name mapping in the scala-http4s generator so encoders/decoders use the JSON base field name instead of the Scala-safe name, including nested oneOf cases. This makes generated models correctly read and write discriminator fields when nameMappings are used. Fixes #23792.

• Bug Fixes
  • Use propertyBaseName for discriminator lookups and writes in model.mustache (mapped/unmapped, nested/not nested).
  • Provide parent discriminator via x-parentDiscriminatorBaseName in ScalaHttp4sClientCodegen.java.
  • Result: Decoders read the actual JSON field; Encoders set the expected field.

^{Written for commit f38a62e. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}