Add getter-only configuration binding parity tests

[svick]

Adds the shared reflection/source-generator parity tests requested in this review comment: #131045 (review).

The tests cover get-only nested objects, collections, and abstract-typed properties when their existing values are both non-null and null. They verify that existing instances are bound and null properties are ignored without throwing.

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[Copilot]

Pull request overview

Adds new xUnit coverage to ensure configuration binding behavior for getter-only properties stays consistent (and non-throwing) across the binder implementations, focusing on binding into existing instances vs ignoring null getter-only properties.

Changes:

• Added a new test model type with getter-only nested/collection/abstract-typed properties that can be pre-initialized or left null.
• Added tests verifying binding mutates existing non-null instances and ignores null getter-only properties without throwing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.TestClasses.cs	Adds ClassWithGetterOnlyProperties test type used to exercise getter-only binding scenarios.
src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs	Adds two new [Fact] tests validating binding behavior for getter-only properties when values are initialized vs null.

[Copilot]

Pull request overview

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

Comments suppressed due to low confidence (3)

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.TestClasses.cs:1097

• To make the null-values case a stronger regression test for “skip binding when getter-only property value is null”, it helps if the collection element type can actually fail to bind. With List<string>, virtually any configuration value binds successfully, so a regression that allocates/binds a temporary list (then discards it) would still pass. Consider switching this to List<int> so the test can use non-parsable values and fail if binding is attempted.

            public NestedOptions? Nested { get; }
            public List<string>? Collection { get; }
            public AbstractBase? Abstract { get; }

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs:2965

• If Collection is switched to List<int>, update the non-null case JSON and assertion accordingly; this keeps the test validating that binding into an existing getter-only collection instance actually happened (and didn’t just no-op).

                {
                    "Nested": { "Integer": 1 },
                    "Collection": [ "item" ],
                    "Abstract": { "Value": 2 }
                }

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs:2978

• The null-values test currently uses bindable values for Nested and Collection, so a regression that still tries to bind a temporary instance for null getter-only properties (then discards it) could pass without being detected. Consider using deliberately non-parsable values so the test fails if binding is attempted (this becomes especially effective once Collection is List<int>).

            IConfiguration configuration = TestHelpers.GetConfigurationFromJsonString(
                """
                {
                    "Nested": { "Integer": 1 },
                    "Collection": [ "item" ],
                    "Abstract": { "Value": 2 }
                }
                """);

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Pull request overview

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

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