#23947 Add tests for scala-sttp4-jsoniter client generator

a follow up PR to https://github.com/OpenAPITools/openapi-generator/pull/23801#top

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

---

Summary by cubic

Add UUID, LocalDate, and OffsetDateTime support to the scala-sttp4-jsoniter generator. These types now serialize in ISO formats across query, header, path, and cookie params, with a /fake/misc sample and tests to verify.

• New Features
  • Extend Primitive to include UUID, LocalDate, OffsetDateTime.
  • Add asString with ISO formatters for OffsetDateTime and LocalDate.
  • Switch all serializers (form, header, path, cookie) to use asString.
  • Update petstore spec with /fake/misc (uuid, date-time, date) and generate FakeApi.parameterTypes sample; add tests and README entry.