XAML xmlns simplifications

[StephaneDelcroix]

this is a merge of #28909, #28969, #29203 and #29181

it's probably easier to review independent PRs, but merging them is a nightmare

• fixes XAML xmlns simplifications #28836

[Copilot]

Pull Request Overview

This PR simplifies and unifies the XAML XML namespace definitions across multiple target platforms by updating the XmlnsDefinitionAttribute constructor signature and adding support for implicit XMLNS declarations. Key changes include:

• Changing constructor parameters for XmlnsDefinitionAttribute from (string xmlNamespace, string clrNamespace) to (string xmlNamespace, string target) across PublicAPI files.
• Introducing a new AllowImplicitXmlnsDeclarationAttribute and updating related XAML parsing tasks.
• Adjustments to error messages and XML namespace management in build tasks.

Reviewed Changes

Copilot reviewed 39 out of 43 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Controls/src/Core/PublicAPI/netstandard/PublicAPI.Shipped.txt	Updated XmlnsDefinitionAttribute signature.
src/Controls/src/Core/PublicAPI/net/PublicAPI.Unshipped.txt	Added AllowImplicitXmlnsDeclarationAttribute and related members.
src/Controls/src/Core/PublicAPI/net-windows/*.txt	Updated public API definitions to reflect the new signature and attribute.
src/Controls/src/Core/PublicAPI/net-tizen/*.txt	Updated public API definitions uniformly for Tizen.
src/Controls/src/Core/PublicAPI/net-maccatalyst/*.txt	Updated public API definitions uniformly for MacCatalyst.
src/Controls/src/Core/PublicAPI/net-ios/PublicAPI.Unshipped.txt	Updated iOS public API definitions.
src/Controls/src/Core/PublicAPI/net-android/*.txt	Updated Android public API definitions.
src/Controls/src/Core/Properties/AssemblyInfo.cs	Added a new XmlnsPrefix for xaml.
src/Controls/src/Core/AllowImplicitXmlnsDeclarationAttribute.cs	Introduced new attribute for implicit XMLNS declarations.
src/Controls/src/Build.Tasks/XmlTypeExtensions.cs	Adjusted XML namespace handling and error messaging.
src/Controls/src/Build.Tasks/XamlTask.cs and XamlCTask.cs	Updated parsing logic to work with the new implicit XMLNS functionality.

Files not reviewed (4)

• Microsoft.Maui-vscode.sln: Language not supported
• src/Controls/docs/Microsoft.Maui.Controls/XmlnsDefinitionAttribute.xml: Language not supported
• src/Controls/src/Core/PublicAPI/net-ios/PublicAPI.Shipped.txt: Language not supported
• src/Controls/src/Core/PublicAPI/net/PublicAPI.Shipped.txt: Language not supported

Comments suppressed due to low confidence (1)

src/Controls/src/Build.Tasks/XmlTypeExtensions.cs:51

• The error message contains a duplicated phrase ('Can't add assembly') which may confuse users. Consider removing the duplicate to clarify the message.

throw new BuildException(BuildExceptionCode.InvalidXaml, null, null, $"Protected Xmlns {attribute.XmlNamespace}. Can't add assembly  Can't add assembly {attribute.Target}/{attribute.AssemblyName}.");