add typescript-axios-slim generator and parity tests

[scarf005]

add observe: 'response' to typescript-axios-slim.

• default stays payload body
• observe: 'response' returns AxiosResponse<T> for headers/status
• typed overloads + generic impl preserve slim surface
• docs and parity tests updated

validated:

• ./mvnw -pl modules/openapi-generator -am -Dtest=TypeScriptAxiosSlimParityTest,TypeScriptAxiosClientCodegenTest -Dsurefire.failIfNoSpecifiedTests=false test

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

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

• File the PR against the correct branch: master
• If your PR solves a reported issue, reference it using GitHub's linking syntax.
• If your PR is targeting a particular programming language, @mention the technical committee members.

[wing328]

@scarf005 thanks for the PR. is this ready for review?

Glad to see the PR leverages GPT-5.4 xhigh on opencode.

[scarf005]

hi, will test the PR locally once more tomorrow and mark it as ready.