Remove IList.Contains use from OfType

[stephentoub]

Fixes #119569

[Copilot]

Pull Request Overview

This PR removes the optimization in OfType<T>().Contains() that delegated to IList.Contains() to fix correctness issues when the source contains items that compare equal but are not of the target type, and to handle multidimensional arrays properly.

• Removes the IList.Contains() delegation optimization from OfTypeIterator<TResult>.Contains()
• Adds comprehensive test coverage for multidimensional arrays with OfType()
• Adds test coverage for the type filtering behavior when objects compare equal but are different types

Reviewed Changes

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

File	Description
src/libraries/System.Linq/src/System/Linq/OfType.SpeedOpt.cs	Removes IList.Contains optimization and adds detailed comment explaining why direct iteration is necessary
src/libraries/System.Linq/tests/OfTypeTests.cs	Adds tests for multidimensional arrays and type filtering behavior with custom equality

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[stephentoub]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/17660453305