Add scalafmt plugin to scala-sttp client generator

[YB0y]

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

Part of #23274

PR checklist

• 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
  

  (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.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Add Scalafmt support to the scala-sttp client generator. The generator now writes .scalafmt.conf and enables sbt-scalafmt for consistent formatting, aligning with scala-sttp4 and contributing to #23274.

• New Features
  • Emit root .scalafmt.conf (Scalafmt 3.10.6, Scala 2.13 dialect) and project/plugins.sbt with sbt-scalafmt 2.5.6.
  • Add templates (project/plugins.mustache, scalafmt.mustache) and wire them in ScalaSttpClientCodegen; update scala-sttp and scala-sttp-circe samples.

^{Written for commit 4bad43b. Summary will update on new commits.}

[cubic-dev-ai]

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