Add scalafmt plugin to scala-finch generator

[volcano303]

Adds scalafmt (Scalafmt 3.10.6, Scala 2.12 dialect) and sbt-scalafmt plugin (2.5.6) to the scala-finch 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 to the Scala Finch server generator to auto-format generated code. Ships a .scalafmt.conf and adds the sbt-scalafmt plugin for easy formatting.

• New Features
  • Generate .scalafmt.conf (Scalafmt 3.10.6, Scala 2.12 dialect) via scalafmt.mustache.
  • Append org.scalameta sbt-scalafmt 2.5.6 to project/plugins.sbt.
  • Update Scala Finch sample to include the config and tidy imports.

^{Written for commit 54df45b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 10 files

[wing328]

CI failure not related to this change

i'll take another look at the CI failure tomorrow