Add scalafmt plugin to scala-http4s-server generator

[volcano303]

Adds scalafmt (Scalafmt 3.10.6, Scala 2.13 dialect) and sbt-scalafmt plugin (2.5.6) to the scala-http4s-server generator, following the same pattern as #23273 (scala-sttp4) and #23378 (scala-sttp4-jsoniter).

Partially addresses #23274

cc @clasnake @shijinkui @ramzimaalej @chameleon82 @Bouillie @Fish86

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
  

  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.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks)
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Summary by cubic

Add Scalafmt to the scala-http4s-server generator so new servers ship with a formatter and consistent style out of the box. Partially addresses #23274.

• New Features
  • Generate .scalafmt.conf (Scalafmt 3.10.6, Scala 2.13 dialect) and project/plugins.sbt with org.scalameta:sbt-scalafmt 2.5.6.
  • Honors EXCLUDE_SBT; files are added only when SBT files are generated.
  • Updated scala-http4s-server sample outputs.

^{Written for commit 36f260d. Summary will update on new commits.}

[cubic-dev-ai]

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