[PowerShell][BUG] Fix issue #18427 - Array context not maintained when converting a parameter to JSON

[condorcorde]

This PR aims to fix an issue due to PowerShell pipeline behaviour: when an array of a single element is passed into a pipeline the array context is not preserved.

When the result of the pipeline is the generation of the JSON body of an API request, it causes the server side to receive an unexpected type when an array of a single element is passed to the PowerShell client function. See Issue #18427

The fix forces the array context when the parameter corresponding to the variable $LocalVarBodyParameter is an array type.

PSPetstore endpoints that require an array type are not implemented on the server side, so the fix has been tested on the Connectwise PSA API.

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 
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  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
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• 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

[wing328]

appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/50310234

[wing328]

thanks for the PR. let's give it a try