feat cpp-qt-client: fix cast primitive to string

[lirik90]

QUrl::toPercentEncoding accepts only QString type, but we pass any primitive type to him
It's fix for issue 20001

For validate the work

1. mark non string property as required in openapi file
2. generate client
3. try build it
4. build must pass without any error messages

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/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

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

[wing328]

thanks for the PR

please follow step 3 to update the samples so that the CI can verify the change

[wing328]

ci failure not related to this change:

https://github.com/OpenAPITools/openapi-generator/actions/runs/11606840555/job/32319917402?pr=20000

[wing328]

thanks for the PR, which has been merged