Fix JSON schema nullability for readonly properties

[Youssef1313]

This a simpler fix for #131602 that is specific in JsonSchemaExporter.

Long-term, I think we would need to fix the underlying values of IsGetNullable and IsSetNullable. So this is more of a workaround to take in RC1, and hence keeping the linked issue open.

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[Copilot]

Pull request overview

Adjusts System.Text.Json’s JsonSchemaExporter nullability inference so schema nullability reflects available accessors (getter/setter) rather than treating missing accessors as nullable, and adds regression coverage for get-only/readonly members.

Changes:

• Update schema nullability logic to only consult IsGetNullable when a getter exists, and IsSetNullable when a setter exists.
• Add a regression test covering get-only properties and readonly fields in schema generation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs	Refines nullability decision to ignore setter nullability when no setter delegate exists (and vice versa).
src/libraries/System.Text.Json/tests/Common/JsonSchemaExporterTests.cs	Adds regression test + new POCO to validate non-nullable schema output for get-only properties and readonly fields.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs:452

• IsNullableSchema now correctly ignores IsSetNullable when propertyInfo.Set is null, but the polymorphic path still computes parentPolymorphicTypeIsNonNullable using propertyInfo is { IsGetNullable: false, IsSetNullable: false } (around line ~134). For get-only/read-only members, IsSetNullable is typically true (WriteState == Unknown) even though there is no setter, which means derived schemas can still be made nullable via the !parentPolymorphicTypeIsNonNullable check.

Consider updating the parentPolymorphicTypeIsNonNullable computation to use the same accessor-existence gating as this change (i.e., treat a missing setter as non-nullable rather than consulting IsSetNullable).

                        if (propertyInfo is not null)
                        {
                            return (propertyInfo.Get is not null && propertyInfo.IsGetNullable) ||
                                (propertyInfo.Set is not null && propertyInfo.IsSetNullable);
                        }

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs:451

• The comment above IsNullableSchema still implies setter nullability is considered even when a property has no setter. Since the implementation now gates IsGetNullable/IsSetNullable on accessor presence, please update the comment to match the new semantics (this avoids confusion for get-only properties, which is the scenario this change fixes).

                        if (propertyInfo is not null)
                        {
                            return (propertyInfo.Get is not null && propertyInfo.IsGetNullable) ||
                                (propertyInfo.Set is not null && propertyInfo.IsSetNullable);

[Youssef1313]

@eiriktsarpalis I'm thinking more. Is this whole idea even correct? A property isn't necessarily writeable only via a setter. It could be writeable via a constructor.

public class C
{
    public C(string? s) => S = s ?? string.Empty;

    // get-only property. Non-nullable.
    // But a null is okay to get deserialized.
    // It's not produced by serialization, though.
    public string S { get; }
}