convert all text/* content types to strings in the apache http java client

[aaronforest-wf]

fix https://github.com/OpenAPITools/openapi-generator/issues/18730

PR checklist

• [x] Read the contribution guidelines.
• [x] 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.
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[aaronforest-wf]

Ready for review

Java Apache Http template creators: @harrywhite4 @andrevegas

Java technical committee: @bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @martin-mfg

[wing328]

@aaronforest-wf thanks for the PR, which looks good to me.

will merge if all tests pass.

[aaronforest-wf]

Thank you @wing328!