[csharp][default] Fix handling of nested inheritance properties

[Blackclaws]

This addresses #8849 partly as there is still another bug remaining. However this addresses the handling of nested inheritance which fixes the model classes created. It contains a change to the default codegen code as well as the issue was actually partly rooted much deeper.

Following issues were fixed:

• Adding properties defined on a composed schema to the inheriting child in a chain inheritance setting
• Fixing csharp codegens when the components had names with underscores in them when inheritance is involved

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 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  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*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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[wing328]

cc @OpenAPITools/generator-core-team as the change covers DefaultCodegen.java

[Blackclaws]

@wing328 How about this merge?

[scavarda]

@wing328 Any news?