Add Strict JsonSerializerOptions

[PranavSenthilnathan]

Add JsonSerializerOptions.Strict which:

• Applies the JsonUnmappedMemberHandling.Disallow policy
• Disables AllowDuplicateProperties
• Preserves case sensitive property binding
• Enables both RespectNullableAnnotations and RespectRequiredConstructorParameters settings

Closes #108526

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[PranavSenthilnathan]

@krwq suggested that the default value AllowDuplicateProperties (even without JsonSerializerOptions.Strict) should be false since that's a safer default. This would be a breaking change, so I'm not sure if we can do it. @eiriktsarpalis thoughts?

[Copilot]

Pull Request Overview

This PR introduces a new JsonSerializerOptions.Strict preset that disallows unmapped members, forbids duplicate properties, enforces case-sensitive binding, and respects nullable annotations and required constructor parameters. It adds support for JsonSerializerDefaults.Strict in the core API and source-gen contexts, updates singleton getters, and covers the new behavior with tests.

• Implement JsonSerializerDefaults.Strict enum and JsonSerializerOptions.Strict property
• Update the JsonSerializerOptions constructor and default/web getters to support Strict
• Add unit tests for both runtime and source-generated contexts using Strict defaults

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/OptionsTests.cs	Added tests for JsonSerializerOptions.Strict behavior
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/JsonSourceGenerationOptionsTests.cs	Added source-gen tests for Strict defaults
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs	Added Strict property, updated default/Web getters and constructor
src/libraries/System.Text.Json/ref/System.Text.Json.cs	Exposed JsonSerializerDefaults.Strict and JsonSerializerOptions.Strict in reference assembly
src/libraries/System.Text.Json/Common/JsonSerializerDefaults.cs	Defined the Strict enum value with XML documentation

Comments suppressed due to low confidence (1)

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/OptionsTests.cs:1325

• Add a test case to verify that JsonSerializerOptions.Strict throws a JsonException when an unmapped JSON property is encountered, to fully cover the JsonUnmappedMemberHandling.Disallow behavior.

public static void JsonSerializerOptions_Strict_SerializesWithExpectedSettings()

[eiriktsarpalis]

This would be a breaking change, so I'm not sure if we can do it. @eiriktsarpalis thoughts?

It's too much of a breaking change. It would need to be opt in unfortunately.

[PranavSenthilnathan]

/ba-g This code doesn't affect iOS and those tests are failing for other PR builds as well