Fix nullability analysis of property indexers

[lilinus]

Fixes #101071

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[Copilot]

Pull request overview

Fixes incorrect NullabilityInfoContext results for indexer parameters by ensuring indexer parameter nullability is read from the appropriate accessor (getter/setter) parameter metadata, and adds regression coverage for the reported tuple-property interaction.

Changes:

• Update NullabilityInfoContext.Create(ParameterInfo) to remap indexer ParameterInfo from PropertyInfo to the corresponding getter/setter parameter.
• Add tests covering indexers with and without an additional value-tuple-returning property (regression scenario).

Reviewed changes

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

File	Description
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Reflection/NullabilityInfoContextTests.cs	Adds regression tests validating indexer parameter nullability states, including the tuple-property scenario.
src/libraries/System.Private.CoreLib/src/System/Reflection/NullabilityInfoContext.cs	Fixes indexer-parameter handling by switching to accessor parameters and respecting nullablePublicOnly behavior.