Add System.Text.Json union support

[eiriktsarpalis]

Summary

Implements System.Text.Json support for C# union-like shapes:

• adds union contract metadata (JsonTypeInfoKind.Union, JsonUnionCaseInfo, JsonUnionInfoValues<T>, union constructor/deconstructor hooks)
• adds JsonUnionAttribute, JsonTypeClassifier, and classifier factory/context APIs for union and polymorphic type classification
• adds union converter support for reflection and source-generated metadata
• teaches the source generator to discover union case constructors and emit union metadata
• adds reflection, source-generation, and source-generator unit coverage for union and structural classifier scenarios

Fixes #127299.

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Pull request overview

Adds System.Text.Json support for C# union-like types, including runtime metadata, custom JSON type classifier APIs, converter support, source-generation plumbing, diagnostics, schema handling, and test coverage.

Changes:

• Adds union metadata/converter infrastructure and classifier APIs for unions and polymorphic types.
• Extends source generation to emit union and polymorphism/classifier metadata plus diagnostics.
• Adds reflection/source-generation tests and updates generated baselines/resources.

Reviewed changes

Copilot reviewed 163 out of 163 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
src/libraries/System.Text.Json/src/System.Text.Json.csproj	Includes new union/classifier source files and shared compiler attributes.
src/libraries/System.Text.Json/ref/System.Text.Json.csproj	Adds shared compiler attribute references for ref build.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Attributes/JsonUnionAttribute.cs	Adds union customization attribute.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Attributes/JsonPolymorphicAttribute.cs	Adds polymorphic classifier hook.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonTypeClassifier*.cs	Adds classifier delegate, context, factory, and kind APIs.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonValueType.cs	Adds internal JSON value-shape flags.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Union/*	Adds union converter and converter factory.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/*	Advertises supported JSON value shapes for built-in converters.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/*Union*	Adds union metadata values and reflection/source-gen metadata support.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo*.cs	Adds union constructor/deconstructor, union cases, classifier resolution, and kind support.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonObjectInfoValuesOfT.cs	Adds generated polymorphism/classifier metadata slots.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonCollectionInfoValuesOfTCollection.cs	Adds generated polymorphism/classifier metadata slots for collections.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonMetadataServices*.cs	Adds union creation and generated polymorphism metadata wiring.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/DefaultJsonTypeInfoResolver*.cs	Adds reflection resolver union discovery and classifier wiring.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/MemberAccessor*.cs	Adds constructor/getter helpers used by union metadata.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions*.cs	Adds classifier list option and cache/equality integration.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.HandleMetadata.cs	Adds polymorphic classifier dispatch during metadata handling.
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverter*.cs	Extends converter strategy/value-shape and classifier polymorphic handling.
src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs	Adds schema generation for union contracts.
src/libraries/System.Text.Json/src/Resources/Strings.resx	Adds runtime union/classifier error messages.
src/libraries/System.Text.Json/Common/JsonSourceGenerationOptionsAttribute.cs	Adds source-generation classifier option.
src/libraries/System.Text.Json/gen/*	Adds source-generator model, diagnostics, parser/emitter, resources, and target updates for unions/classifiers.
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/*	Adds reflection serializer union/classifier test wrappers and project includes.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/*	Adds source-generation union/classifier/polymorphism tests and project includes.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/*	Adds diagnostics/output support and baseline updates.
docs/project/list-of-diagnostics.md	Documents new SYSLIB1227/SYSLIB1228 diagnostics.

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