[C++] [Client] Fixes to cpp-oatpp-server generator.

[Kraust]

When testing #21547 ran into a few issues that didn't make the API compliant:

• Model Fields were the wrong case
• Enum Fields weren't properly supported.

I've fixed both of those inconsistencies. It did not register to me that petstore does not make use of enums.

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.

@ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08) @aminya (2025/05)

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FYI: I still have interest in creating the oat++ client, that may be on my list of things to do next week.

[wing328]

FYI. I just merged #21601 to have cpp oatpp server using its own test spec: modules/openapi-generator/src/test/resources/3_0/cpp-oatpp-server/petstore.yaml

please feel free to update it with more test schemas and endpoints (e.g. tags: fake) to cover more cases