fix(go-server): avoid AssertstringRequired for primitive-typed models

[halfcrazy]

oneOf/nullable enums can flatten to string while still flagged as models, so templates must not emit Assertstring helpers that do not exist.

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Summary by cubic

Fixes go-server codegen to stop emitting Assertstring* helper calls for primitive-typed models (e.g., nullable oneOf enums). Prevents invalid references and compile errors in generated code.

• Bug Fixes
  • Detects Go primitives and clears isModel to avoid generating non-existent Assertstring* helpers.
  • Updates model.mustache to skip Assert* calls when a property (or its items) is primitive.
  • Adds tests and a sample spec to verify no AssertstringRequired/Constraints are generated and AssertThingRequired is a no-op.

^{Written for commit 0b652fe. Summary will update on new commits.}

[halfcrazy]

cc @antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04) @lwj5 (2023/04)

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}