[Foundation] Fix nullability in NSMutableSet.

[rolfbjarne]

This is file 31 of 47 files with nullability disabled in Foundation.

Changes:

• Enabled nullable reference types
• Removed 'To be added' placeholder comments
• Added comprehensive XML documentation for constructors and operators
• Made operator parameters nullable to match actual usage patterns
• Made subtraction operator return type nullable since it returns null for empty/null first operand
• Added see cref attributes for better documentation
• Add extensive tests.

Contributes towards #17285.

[Copilot]

Pull request overview

This pull request enables nullable reference types for NSMutableSet as part of a broader effort to enable nullability across the Foundation namespace. The changes improve code safety by making null handling explicit in operator overloads and replace placeholder documentation with comprehensive XML comments.

Key Changes:

• Enabled nullable reference types by adding #nullable enable directive and adding proper nullable annotations
• Replaced "To be added" placeholder documentation with detailed XML documentation for constructors and operators
• Made operator parameters and return types nullable with appropriate NotNullIfNotNull attributes
• Added 17 comprehensive unit tests covering all null and empty set scenarios for both addition and subtraction operators

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Foundation/NSMutableSet.cs	Enabled nullable reference types, added XML documentation for constructors and operators, made operator signatures nullable-aware
tests/monotouch-test/Foundation/NSMutableSetTest.cs	Added 17 new unit tests covering edge cases for + and - operators including null, empty, and overlapping set scenarios
tests/cecil-tests/Documentation.KnownFailures.txt	Removed documentation failure entries for the two operators now that they have proper XML documentation

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Test results

✅ All tests passed on VSTS: test results.

🎉 All 117 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 9 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 9 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 9 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

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