#23300 Add scalafmt (plugin) to scala-http4s client generator

Summary

• Adds scalafmt.conf and sbt-scalafmt plugin to the scala-http4s client generator for consistent code formatting
• Uses runner.dialect = scala3 to match the generator's Scala 3.6.4 target
• Follows the pattern established in #23273 for scala-sttp4

Refs #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.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/scala-http4s.yaml || exit
  

  Commit all changed files.
• File the PR against the correct branch: master
• If your PR solves a reported issue, reference it using GitHub's linking syntax
• 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-http4s client generator for consistent formatting of generated Scala 3 code. Generates the plugin and config so users can run formatting out of the box.

• New Features
  • Generate .scalafmt.conf and project/plugins.sbt for scala-http4s clients.
  • Include sbt-scalafmt plugin 2.5.6 and Scalafmt version=3.10.6 with runner.dialect = scala3 (matches Scala 3.6.4 target).
  • Only added when SBT files are generated (skipped if excludeSbt is true).