JSON: Don't mark unmapped properties as read

[PranavSenthilnathan]

During deserialization we keep track of the properties that are read in order to do duplicate detection and required property validation. When a parametrized constructor is specified for deserialization, there are four types of JSON properties:

1. Properties that map to a constructor parameter
2. Properties that don't map to a constructor parameter but still map to a property on the class
3. Properties that map to the ExtensionData property
4. Properties that are unmapped

(The last two cases are only differentiated by whether the class has a property with the ExtensionData attribute.)

#115856 added duplicate detection by keeping track of properties that were assigned. Paths to handle cases 1, 2 and 3 were added but case 4 was an edge case that was not considered. So when this case was hit we would try to find the mapping like we do for case 1 or 2 when we actually don't have one. This PR fixes this case by explicitly checking for it and skipping duplicate detection for it.

Fixes #116801

[Copilot]

Pull Request Overview

This PR fixes an issue where unmapped JSON properties were incorrectly being marked as "read" during deserialization, which affected duplicate detection and required property validation. Key changes include:

• Updating tests to include the new behavior with a class having a required property.
• Modifying the converter logic to skip marking unmapped properties as read.
• Adding corresponding test cases to verify the fix.

Reviewed Changes

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

File	Description
System.Text.Json.SourceGeneration.Tests/Serialization/ConstructorTests.cs	Added JsonSerializable attribute for ClassWithRequiredProperty in both metadata and default contexts.
ConstructorTests.ParameterMatching.cs	Added a new test case to ensure unmapped properties do not interfere with duplicate detection.
ObjectWithParameterizedConstructorConverter.cs	Adjusted condition and comment to prevent marking unmapped properties as read.

Comments suppressed due to low confidence (2)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Object/ObjectWithParameterizedConstructorConverter.cs:632

• [nitpick] Consider enhancing the comment to reference issue JsonSerializer.Deserialize() may throw NullReferenceException for unknown properties #116801 for additional context on why unmapped properties are skipped during duplicate detection.

            if (!useExtensionProperty && jsonPropertyInfo != JsonPropertyInfo.s_missingProperty)

src/libraries/System.Text.Json/tests/Common/ConstructorTests/ConstructorTests.ParameterMatching.cs:1825

• [nitpick] Consider extending the 'RequiredMemberWithUnmappedMember' test to explicitly validate that duplicate detection is bypassed for unmapped properties, if such verification is feasible.

        [Fact]

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[eiriktsarpalis]

Thanks for the fast fix @PranavSenthilnathan

[PranavSenthilnathan]

/ba-g Known test failures, unrelated to this PR. #116671 and #116558

[joperezr]

Thanks for fixing this @PranavSenthilnathan! Do you plan on backporting this to preview 6? We need it in order to be able to get the new aspnetcore Passkey feature working again

[PranavSenthilnathan]

/backport to release/10.0-preview6

[github-actions]

Started backporting to release/10.0-preview6: https://github.com/dotnet/runtime/actions/runs/15763591698