[scala-sttp4] Fix enum operation parameters: type import and wire-value toString

[vivekmahajan]

Fixes #24347.

Enums used as operation parameters in the scala-sttp4 generator are broken in two ways:

1. Generated code does not compile. The API imports enum types Enumeration-style (import pkg.MyEnum._), inherited from the sttp3 generator where type MyEnum = Value lives inside the companion. scala-sttp4 enums are sealed traits + case objects, so the wildcard import does not bring the type into scope:

import org.openapitools.client.model.PetStatus._   // does not import the trait
def getPetsByStatus(status: PetStatus): Request[...] // error: not found: type PetStatus

Fixed by importing the type itself. Same-package model imports need no import at all for sealed traits, so that special case is removed along with the now-unused enumRefs / isEnumClass / getEnumRefs helpers.

2. Path/query interpolation serializes the Scala identifier, not the wire value. uri"$baseUrl/pets/${status}" uses toString, so a request for sold-out goes out as /pets/SoldOut. Fixed by overriding toString on each enum case object with its wire value, matching the sttp3 generator's behavior (Enumeration.Value("sold-out").toString is the wire value there):

case object SoldOut extends PetStatus { override def toString: String = "sold-out" }

JSON codecs are unaffected — both circe and json4s map enum values explicitly (visible in the regenerated samples).

Tests: Sttp4CodegenTest#verifyEnumParameterImportAndWireValues with a new spec fixture (sttp4-enum-param.yaml): a string enum (including a hyphenated value) used as a path parameter and as a model property. Asserts the plain type import in the API, no wildcard import, no same-package import in models, and wire-value toString on case objects.

Also verified end-to-end against a real FastAPI-generated spec (enum path param + discriminated oneOf bodies): the generated client compiles and produces correct URLs, correct request bodies, and round-trips responses.

scala-sttp4-jsoniter is unaffected (separate codegen class and templates); its samples are unchanged by regeneration.

PR checklist

• Read the contribution guidelines.
• Built the project and updated samples: ./bin/generate-samples.sh bin/configs/scala-sttp4*.yaml — sample changes are committed. No generator options changed, so no doc export changes.
• Filed against master.
• Scala technical committee: @clasnake @jimschubert @shijinkui @ramzimaalej @chameleon82 @Bouillie

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Summary by cubic

Fixes enum parameters in scala-sttp4 so generated clients compile and send wire values in URLs. APIs now import the enum type directly, and enum case objects stringify to their wire values.

• Bug Fixes

  • Import the enum type in APIs (no MyEnum._ wildcard; no same-package model imports).
  • Enum case objects override toString with the wire value (path/query params serialize correctly, including nested ...Enums).

• Refactors

  • Removed enum-specific import helpers and simplified import processing.
  • Added a focused test and spec fixture; regenerated scala-sttp4 and scala-sttp4-circe samples. scala-sttp4-jsoniter unchanged.

^{Written for commit dcd86c8. Summary will update on new commits.}