Add scalafmt plugin to scala-sttp4-jsoniter client generator

[YB0y]

Adds scalafmt (Scalafmt 3.10.6, Scala 2.13 dialect) and sbt-scalafmt plugin (2.5.6) to the scala-sttp4-jsoniter client generator, following the same pattern as #23273 (scala-sttp4).

Part of #23274

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• 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
  

  Commit all changed files.
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Summary by cubic

Adds Scalafmt support to the scala-sttp4-jsoniter client generator for consistent Scala formatting across generated projects. Regenerates samples after rebase. Part of #23274.

• New Features
  • Generator emits .scalafmt.conf (Scalafmt 3.10.6, Scala 2.13) and adds sbt-scalafmt plugin (2.5.6) via project/plugins.sbt using scalafmt.mustache and project/plugins.mustache.
  • Updated Petstore scala-sttp4-jsoniter sample to include the new files; other samples refreshed by regen.

^{Written for commit b6c44f8. Summary will update on new commits.}

[YB0y]

Hi @wing328 Could you please help me with passing CI test?
How can I address Samples up-to-date CI test failure?

[wing328]

don't worry. i'll take care of it later

[YB0y]

@wing328 Sorry for the ping. When you have time, could you take a look at this? I’d appreciate your help getting the CI tests passing and moving this toward a merge.