[Foundation] Improve nullability in NSArray.ToArray.

[rolfbjarne]

• Enable #nullable enable file-wide in NSArray.cs.
• Improve nullability annotations for ToArray (return TKey?[]).
• Improve nullability annotations for NSArray.ToArray (return TKey?[]).
• Add NonNullToArrayDropNullElements instance methods for callers needing non-null arrays.
• Update CVPixelBufferAttributes.PixelFormatTypes to handle nullable ToArray elements.
• Update NSPasteboard/NSPasteboardItem DetectValues Wrap attributes to use NonNullToArrayDropNullElements.
• Update XML docs for all modified members.

Contributes towards #17285.

[Copilot]

Pull request overview

This PR enables nullable reference types in Foundation.NSArray and adjusts ToArray APIs to correctly represent NSNull values as null in managed arrays, while adding drop-null helpers for callers that require non-null elements. It also updates a few call sites to account for the new nullable element types and removes related documentation known-failures.

Changes:

• Enable #nullable in src/Foundation/NSArray.cs and update ToArray/NSArray<TKey>.ToArray to return nullable-element arrays.
• Add NonNullToArrayDropNullElements helpers and update AppKit wrap code to use them.
• Update CoreVideo pixel format conversion to compile with nullable NSArray.ToArray() elements and adjust documentation known-failures.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/cecil-tests/Documentation.KnownFailures.txt	Removes known-failure entries for updated NSArray.ToArray XML docs.
src/appkit.cs	Updates DetectValues wrappers to drop null elements when converting NSArray values to managed arrays.
src/Foundation/NSArray_1.cs	Updates generic NSArray<TKey>.ToArray() signature/docs to return TKey?[].
src/Foundation/NSArray.cs	Enables nullable and updates ToArray nullability; introduces non-null drop-null conversion helpers.
src/CoreVideo/CVPixelBufferAttributes.cs	Adjusts PixelFormatTypes conversion to handle nullable NSArray.ToArray() elements.

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✅ 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
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✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 6 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ monotouch (iOS): All 11 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 11 tests passed. Html Report (VSDrops) Download
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