Adds a SchemaUtils class with common schema operations. Improves readability and makes the if-cases easier to read as the method name clearly highlights what is checked.

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Summary by cubic

Centralizes schema checks in ModelUtils and refactors generators/utilities to use them for consistent, clearer logic. No functional changes intended.

• Refactors
  • Added helpers in org.openapitools.codegen.utils.ModelUtils: hasAllOf, hasOneOf, hasAnyOf, hasProperties, hasEnum, isObjectTypeOAS30.
  • Replaced inline checks across DefaultCodegen, InlineModelResolver, OpenAPINormalizer, language codegens (AbstractJavaCodegen, AbstractPhpCodegen, AbstractPythonCodegen, AbstractPythonPydanticV1Codegen, CppHttplibServerCodegen, CppRestSdkClientCodegen), and OpenApiSchemaValidations.
  • C++ server: added local isNestedObject and standardized oneOf/anyOf handling using std::variant.

^{Written for commit 79df5f5. Summary will update on new commits.}