[BUG] [pistache] models don't compile due to calling `validate()` with no parameters not returning bool

This PR introduced a change to call if (!m_{{name}}.validate()) { but the no-argument validate function throws on errors and returns void, this causes a compiler error:

error: could not convert '((const org::openapitools::Model1*)this)->org::openapitools::Model1::m_Parameters.org::openapitools::Model2::validate()' from 'void' to 'bool'
   47 |     if (!m_Parameters.validate()) {
      |          ~~~~~~~~~~~~~~~~~~~~~^~
      |                               |
      |                               void
openapi/model/Model1.cpp:47:32: error: in argument to unary !
   47 |     if (!m_Parameters.validate()) {
      |                                ^

Found in OpenAPI 7.0.1 with the following command:

npx @openapitools/openapi-generator-cli generate -i api.yml -g cpp-pistache-server -o out

Reproduce the issue with the shown npx @openapitools/openapi-generator-cli command, using the cpp-pistache-server generator and api.yml. Inspect the generator templates responsible for model validation and verify that generated Model1 and Model2 sources compile successfully without the reported void-to-bool error.