fix: honour OAS required and nullable flags (#3911)

[OMMHOA]

Implemented with heavy use of AI, manually reviewed the change. AI summary of the change:

Issue #3911 — Implementation Summary

Fixes Kiota generating all model properties as nullable regardless of the OAS required array and nullable flag.

Rule implemented: a property is marked non-nullable only when it is both listed in the parent schema's required array and its own schema does not carry nullable: true. All other combinations remain nullable (the safe default).

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Files changed

src/Kiota.Builder/CodeDOM/CodeProperty.cs (previous session)

• Added bool IsRequired { get; set; } property.
• Updated Clone() to copy the field.

src/Kiota.Builder/Extensions/OpenApiSchemaExtensions.cs

• Added internal static bool IsExplicitlyNullable(this IOpenApiSchema? schema) helper that detects OAS 3.0 nullable: true and OAS 3.1 type: null / anyOf [{type: null}] patterns.

src/Kiota.Builder/KiotaBuilder.cs

• CreatePropertiesForModelClass — reads schema.Required and passes isRequired to each CreateProperty call.
• CreateProperty — added bool isRequired = false parameter; sets prop.IsRequired = isRequired on every property; sets prop.Type.IsNullable = false when the property is required and IsExplicitlyNullable() returns false. Clones existingType before mutation to avoid corrupting shared type references.

src/Kiota.Builder/Refiners/CSharpRefiner.cs

• MakeEnumPropertiesNullable — added .Where(x => !x.IsRequired) guard so required enum properties are no longer forced nullable.

src/Kiota.Builder/Writers/CSharp/CodePropertyWriter.cs

• isNullableReferenceType check — added && codeElement.Type.IsNullable so the #nullable enable / ? block is only emitted when the type is actually nullable. Required non-nullable properties are written as plain public T Name { get; set; }.

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Tests added

tests/Kiota.Builder.Tests/KiotaBuilderTests.cs

7 new tests in #region Issue-3911 verifying IsNullable and IsRequired on the Code DOM for:

Schema type	required	nullable	Expected IsNullable	Expected IsRequired
string	✅	❌	false	true
string	✅	✅	true	true
string	❌	❌	true	false
string	❌	✅	true	false
integer	✅	❌	false	true
object $ref	✅	❌	false	true
enum $ref	✅	❌	false	true
array	✅	❌	false	true

tests/Kiota.Builder.Tests/Writers/CSharp/CodePropertyWriterTests.cs

• Required non-nullable reference type → no #nullable enable, no ? suffix.
• Nullable reference type → has #nullable enable and ? suffix.
• Required non-nullable enum → no #nullable enable, no ? suffix.

tests/Kiota.Builder.Tests/Writers/Java/CodePropertyWriterTests.cs

• Required non-nullable → @jakarta.annotation.Nonnull, no @Nullable.

tests/Kiota.Builder.Tests/Writers/Php/CodePropertyWriterTests.cs

• Required non-nullable → no ? type prefix, no = null default.

tests/Kiota.Builder.Tests/Writers/Python/CodePropertyWriterTests.cs

• Required non-nullable → no Optional[...], no = None default.

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Languages requiring no code changes

Java, PHP, and Python writers already consumed IsNullable faithfully and produce correct output as soon as the builder sets the flag correctly. Go, TypeScript, and Ruby were unaffected by design.

[OMMHOA]

@microsoft-github-policy-service agree

[WolfgangHG]

Just saw this issue, and a alarm bell rings ;-): some time ago, I run into a "required" problem with a NSwag generated client: I had to build an api client and the openapi spec was not not correct and did not define fields as optional.
The reason was probably that it was converted from Swagger 2 to OpenApi 3 and the required/optional definitions were not checked.

After re-generating the client, it failed to parse e.g. server responses because fields missing in the response were considered be required.

This might be a sample class (not exactly the property that caused the problem originally):

    SampleModel:
      type: object
      properties:
        field:
          type: number

With the switch described below, NSwag generates this (working) nullable property:

[Newtonsoft.Json.JsonProperty("field", Required = Newtonsoft.Json.Required.Default, NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore)]
public double? Field { get; set; }

With default settings, it would create a non-nullable property:

[Newtonsoft.Json.JsonProperty("field", Required = Newtonsoft.Json.Required.DisallowNull, NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore)]
public double Field { get; set; }

Could this happen here? If yes: could you add a switch to use the old way?

NSwag fortunately had a switch for this - https://www.github.com/RicoSuter/NSwag/issues/2313#issuecomment-516471348