[csharp] unity error message and code cleanup

[balassit]

CSharpClientCodegen.cs

• use final keyword for getLibrary() within a method
• include unity in error message when unsupported library is provided by converting of/else to a map
• convert if/else to switch
• replace hard coded strings with variable usage
• postProcessModels -> simplify to avoid redundant checks
  • Boolean.FALSE.equals() explicitly checks for false, but noneMatch() already returns true when no match is found. This avoids unnecessary boxing/unboxing of Boolean values.
  • removed the !cm.getComposedSchemas().getOneOf().isEmpty() check because removeIf() inherently handles an empty list safely.

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 Git BASH)
  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]

cc @mandrean (2017/08) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05) @iBicha (2023/07)

[wing328]

also cc @devhl-labs (author of generichost)