[UIKit] Enable nullability and clean up UIDragDropSessionExtensions.

[rolfbjarne]

This is file 8 of 30 files with nullability disabled in UIKit.

• Enable nullability (#nullable enable).
• Use ArgumentNullException.ThrowIfNull() for parameter validation.
• Add nullable annotation (T[]?) for completion handler parameter to reflect that the array can be null.
• Improve XML documentation comments: remove 'To be added.' placeholders, add proper descriptions for all parameters/typeparams/returns, add 'see cref' references, remove extra whitespace, reorder elements (summary first), remove empty remarks elements.

Contributes towards #17285.

[Copilot]

Pull request overview

This PR enables nullable reference types for the UIDragDropSessionExtensions class in UIKit and improves code quality through better documentation and null handling.

Changes:

• Enabled nullable reference types by adding #nullable enable directive
• Added null argument validation using ArgumentNullException.ThrowIfNull()
• Enhanced XML documentation comments by removing placeholder text and adding detailed descriptions

Comments suppressed due to low confidence (1)

src/UIKit/UIDragDropSessionExtensions.cs:38

• The GC.KeepAlive (v) call should be moved after the completion (arr) call on line 40. The purpose of GC.KeepAlive is to ensure the object remains alive until that point in execution, but currently it's called before the completion handler which may still reference the object.

				GC.KeepAlive (v);

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NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

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💻 [CI Build #f2c55c7] Tests on macOS M1 - Mac Monterey (12) passed 💻

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🚀 [CI Build #f2c55c7] Test results 🚀

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✅ 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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