[UIKit] Enable nullability and clean up UICollectionView.

[rolfbjarne]

This is file 5 of 30 files with nullability disabled in UIKit.

• Enable nullability (#nullable enable).
• Remove unused System.Threading.Tasks using directive.
• Use ArgumentNullException.ThrowIfNull() instead of manual null checks.
• Add nullable annotation for Source property (UICollectionViewSource?).
• Simplify Source property getter with expression body.
• Replace include file references with inlined XML documentation.
• Remove inlined DocId entries from docs/api/UIKit/UICollectionView.xml.
• Improve XML documentation comments: remove 'To be added.' placeholders, fix whitespace, add missing docs, add 'see cref' references.

Contributes towards #17285.

[Copilot]

Pull request overview

Enables nullable reference types for UICollectionView and refreshes related API/documentation to reduce “nullable-disabled” surface area in UIKit.

Changes:

• Enabled #nullable in src/UIKit/UICollectionView.cs and updated signatures/docs (notably Source and registration helpers).
• Updated ObjCRuntime.Class.GetHandle to accept nullable Type and return NativeHandle.Zero for null.
• Removed now-redundant XML DocId blocks from docs/api/UIKit/UICollectionView.xml after inlining docs into source.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/UIKit/UICollectionView.cs	Enables nullability and updates helper APIs + XML docs for UICollectionView.
src/ObjCRuntime/Class.cs	Adjusts Class.GetHandle nullability/behavior to support nullable call sites.
docs/api/UIKit/UICollectionView.xml	Deletes DocId sections that are no longer referenced after doc inlining.

[Copilot]

RegisterClassForCell (Type? cellType, NSString reuseIdentifier) declares cellType as nullable and the XML remarks explicitly say callers may pass null to un-register, but the method now throws via ArgumentNullException.ThrowIfNull (cellType). This makes the API contract inconsistent and prevents un-registration. Either allow null by removing the throw (and passing a zero handle), or make cellType non-nullable and update the remarks accordingly.

Suggested change

	ArgumentNullException.ThrowIfNull (cellType);
	RegisterClassForCell (Class.GetHandle (cellType), reuseIdentifier);
	var handle = cellType is null ? System.IntPtr.Zero : Class.GetHandle (cellType);
	RegisterClassForCell (handle, reuseIdentifier);

[Copilot]

The remarks say callers may pass null for reuseIdentifier, but reuseIdentifier is non-nullable here; based on the surrounding text this should refer to cellType instead. Please fix the XML documentation so it matches the actual parameters/behavior.

Suggested change

/// <para>The application developer may pass <see langword="null" /> as the <paramref name="reuseIdentifier" />, in which case the <paramref name="cellType" /> will be "un-registered" and no longer instantiated. The application developer may pass in a <paramref name="reuseIdentifier" /> previously associated with another type, in which case the old type will be "de-registered" and the new <paramref name="cellType" /> will be used.</para>

/// <para>The application developer may pass <see langword="null" /> as the <paramref name="cellType" />, in which case the <paramref name="cellType" /> will be "un-registered" and no longer instantiated. The application developer may pass in a <paramref name="reuseIdentifier" /> previously associated with another type, in which case the old type will be "de-registered" and the new <paramref name="cellType" /> will be used.</para>

[Copilot]

The remarks state callers may pass null for reuseIdentifier, but reuseIdentifier is non-nullable; this appears to mean nib (which is nullable) can be null to un-register. Please correct the XML docs to avoid misleading API consumers.

Suggested change

/// <para>The application developer may pass <see langword="null" /> as the <paramref name="reuseIdentifier" />, in which case the <paramref name="nib" /> will be "un-registered" and no longer instantiated. The application developer may pass in a <paramref name="reuseIdentifier" /> previously associated with another nib, in which case the old nib will be "de-registered" and the new <paramref name="nib" /> will be used.</para>

/// <para>The application developer may pass <see langword="null" /> as the <paramref name="nib" />, in which case the <paramref name="nib" /> will be "un-registered" and no longer instantiated. The application developer may pass in a <paramref name="reuseIdentifier" /> previously associated with another nib, in which case the old nib will be "de-registered" and the new <paramref name="nib" /> will be used.</para>

[Copilot]

Class.GetHandle now accepts null and returns NativeHandle.Zero for null input, which is a behavioral change (previously null would fail). This can hide programming errors and may have broader impact beyond the intended un-registration scenarios. Consider keeping the existing throwing behavior and introducing a separate helper (or clearly documenting the new null->Zero behavior in the XML docs).