Fix handling of top-level nullable type schema generation.

[eiriktsarpalis]

Fix #117493.

[Copilot]

Pull Request Overview

This PR fixes the schema exporter to correctly treat top-level nullable types (Nullable<T>) as nullable and adds tests for primitive nullable schemas.

• Registers DateTimeOffset? in the source-gen serializer for top-level schema generation
• Introduces int? and DateTimeOffset? test cases in JsonSchemaExporterTests.TestTypes.cs
• Refactors CompleteSchema to factor out nullable-detection logic and include Nullable<T> support

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSchemaExporterTests.cs	Added [JsonSerializable(typeof(DateTimeOffset?))] to enable source-gen schema support for nullable DateTimeOffset
src/libraries/System.Text.Json/tests/Common/JsonSchemaExporterTests.TestTypes.cs	Added TestData<int?> and TestData<DateTimeOffset?> cases to verify primitive nullable schema output
src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs	Refactored CompleteSchema to a local IsNullableSchema function that now special-cases Nullable<T>

Comments suppressed due to low confidence (1)

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSchemaExporterTests.cs:44

• Add a corresponding test in the source-generation suite that asserts a DateTimeOffset? schema has type ["string","null"] and format "date-time" so the new registration is actually verified.

        [JsonSerializable(typeof(DateTimeOffset?))]