refactor: add utility methods for allOf, oneOf and anyOf CodegenModel checks

[Mattias-Sehlstedt]

Similar to the changes done in #23608 where the available util methods for Schema was used further. This PR introduces and uses the same structure but for CodegenModel and handling the presence of oneOf/anyOf/allOf.

PR checklist

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Summary by cubic

Adds ModelUtils helpers to check allOf/oneOf/anyOf on CodegenModel, and refactors generators to use them. This unifies composed schema checks and reduces null/empty guard code without changing behavior.

• Refactors
  • Added ModelUtils.hasAllOf(CodegenModel), ModelUtils.hasOneOf(CodegenModel), ModelUtils.hasAnyOf(CodegenModel).
  • Replaced manual checks across generators: Go (AbstractGoCodegen, GoClientCodegen), Python (AbstractPythonCodegen, AbstractPythonPydanticV1Codegen, PythonClientCodegen), Rust, C# (CSharpClientCodegen, CSharpReducedClientCodegen), Java (JavaClientCodegen), Kotlin (KotlinServerCodegen), OCaml, PowerShell, Protobuf, and PlantUML.
  • Cleaned up spots where nullability, imports, and validators depend on oneOf/anyOf presence. No functional changes intended.

^{Written for commit e73096d. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 15 files

_{Re-trigger cubic}