#24374 fix(go-server): avoid AssertstringRequired for primitive-typed models

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

Stops Go server codegen from emitting Assertstring* calls when composed schemas flatten to Go primitives (e.g., nullable oneOf enums), preventing bad references and compile errors. Primitive detection is centralized in AbstractGoCodegen so all Go generators treat these as primitives.

• Bug Fixes
  • Clear isModel and set isPrimitiveType in AbstractGoCodegen.fromProperty when the Go type (incl. pointer types) is primitive; applies to array items too.
  • Add missing Go builtins to languageSpecificPrimitives: uint8, uint16, int8, int16, uintptr, and update Go generator docs to list them.
  • Tests and sample spec verify no AssertstringRequired/Constraints generation and AssertThingRequired is a no-op.