Add scalafmt plugin to scalatra generator

[volcano303]

Adds scalafmt (Scalafmt 3.10.6, Scala 2.12 dialect) and sbt-scalafmt plugin (2.5.6) to the scalatra generator, following the same pattern as #23273 (scala-sttp4) and #23378 (scala-sttp4-jsoniter).

The sbt-scalafmt plugin line is appended to the existing project/plugins.sbt.

Partially addresses #23274

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

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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.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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Summary by cubic

Add Scalafmt support to the Scalatra server generator by generating .scalafmt.conf and enabling the sbt-scalafmt plugin, so new projects are ready for auto-formatting. Updates the Scalatra sample to include the config and plugin.

• New Features

  • Generate .scalafmt.conf from scalafmt.mustache (Scalafmt 3.10.6, Scala 2.12 dialect).
  • Append org.scalameta sbt-scalafmt 2.5.6 to project/plugins.sbt in the template and sample.
  • Register the new config file in ScalatraServerCodegen via SupportingFile.

• Migration

  • No breaking changes. New Scalatra projects include the formatter; run sbt scalafmtAll to format.

^{Written for commit 2159b6b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 12 files