[C++][Pistache] Compile error when nesting component/schema reference objects (#16711)

[keyjh2]

When components/schema/ reference other objects, validate() was getting called on the referenced objects with no arguments. The return value was void, but checked as if it was a boolean value.

• Added message arguments to fixed compiler error in model-source.mustache.
• Added integration test for this issue (bin/configs/cpp-pistache-server-cpp-pistache-nested-schema-refs.yaml).
• Added new schema for integration test (modules/openapi-generator/src/test/resources/3_0/nested-schema-refs.yaml).
• Added generated code from integration test (samples/server/petstore/cpp-pistache-nested-schema-refs/)
• Removed unused model source files from previous integration test (samples/server/petstore/cpp-pistache/model).
• Removed pistache install script. Pistache was already getting built as a dependency by CMake.

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[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/8977477159/job/24661842254?pr=18586 already fixed in the master

[wing328]

thanks for the PR. let's give it a try