fix: infer omitted enum schema types

[geirsagberg-udir]

Summary

• infer string when every non-null enum value is a string
• infer integer when every non-null enum value is an integral value supported by the generated C# enum
• leave mixed, fractional, empty, and otherwise ambiguous enums untyped
• preserve explicit OpenAPI type as authoritative

Why

ASP.NET Core with JsonStringEnumConverter can emit component schemas with string enum values but no type (dotnet/aspnetcore#61303). These currently fall through to an empty record. OpenAPI Generator 7.25.0 and NSwag 14.7.1 both infer a string enum for the same schema.

A schema containing only enum is valid, but enum values can technically mix JSON types. Inference is therefore limited to homogeneous values that map cleanly to the generator’s supported C# enum representations.

Verification

• focused tests cover typeless string and integer enums plus mixed and fractional ambiguity
• dotnet build --no-restore --configuration Release
• dotnet test --no-build --configuration Release --verbosity normal (278 passed, 1 intentionally skipped)

[nikcio]

@geirsagberg-udir Thanks for this fix