[ci][elixir] check if formatting is valid

[efcasado]

Description

This pull request builds on #21214 by adding a check to ensure that files generated by the Elixir generator are correctly formatted according to the provided formatter.exs configuration.

All files under /samples/client/petstore/elixir, including test files, have been formatted using mix format.
This improves overall consistency, readability, and maintainability by enforcing standardized formatting across the project.

@wing328, @mrmstn - As usual, curious to hear your thoughts on this one 😊 It does not add any new functionality, but I feel it can help us improve the overall developer experience when it comes to contributing to the project.

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 is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

please have a look at https://github.com/OpenAPITools/openapi-generator/actions/runs/15049963932/job/42302322976?pr=21285 when you've time

for other language/generator, we usually don't check the format in the CI as it's sometimes quite hard format the output in a particular way using mustache templates

we recommend users to run code formatter/linter post code generation (e.g. there's a maven/gradle task to format the auto-generated Java code based on rules in an XML file)

[efcasado]

It seems there might be good reasons why this check was excluded in the first place? The "Samples up-to-date" check runs the bin/generate-samples.sh script, which does not run mix format on the generated samples and I believe updating the script to include mix format may be a no-go since it would require everyone contributing to this project to have Elixir installed locally?

[wing328]

it would require everyone contributing to this project to have Elixir installed locally?

exactly!

[efcasado]

Makes sense! I will close this PR. It turns out is wasn't such a good idea as I initially though 🙈