[X] implicit xmlns declarations

[StephaneDelcroix]

Description of Change

Make the default xmlns implicitely pointing to maui default xmlns pre-load all available XmlsPrefixes for use

Enable this feature by adding this into the .csproj

  <PropertyGroup>
    <DefineConstants>$(DefineConstants);MauiAllowImplicitXmlnsDeclaration</DefineConstants>
    <EnablePreviewFeatures>true</EnablePreviewFeatures>
  </PropertyGroup>

Issues Fixed

• fixes make the default xmlns declaration optional #28849
• fixes pre-defined xmlns prefixes #28850
• fixes Implicit global xmlns #28847 (as this PR introduces a source generator for assembly level random attributes)

[Copilot]

Pull Request Overview

This PR implements support for implicit xmlns declarations by pre-loading available XmlnsPrefix attributes and updating various XAML parsing and code-behind generation routines. Key changes include:

• Adding implicit namespace support in XamlParser, XamlLoader, and related build tasks.
• Refactoring CodeBehindGenerator to incorporate new assembly cache information (including xmlns prefixes) and updated type signatures.
• Updating and adding test cases to verify the new implicit xmlns behavior.

Reviewed Changes

Copilot reviewed 11 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Controls/tests/Xaml.UnitTests/HRTests.cs	Adds a new SetUp method to reset Application.Current for test isolation.
src/Controls/tests/Xaml.UnitTests/GlobalXmlns.xaml.cs	Introduces tests for validating behavior when implicit xmlns declarations are used.
src/Controls/tests/SourceGen.UnitTests/SourceGenXamlTests.cs	Adds tests verifying global namespace handling and behavior when non-XAML is provided.
src/Controls/src/Xaml/XamlParser.cs	Adds GetXmlnsPrefixAttributes to return preloaded xmlns prefixes using new array initializer syntax.
src/Controls/src/Xaml/XamlParser.Namespaces.cs	Introduces DefaultImplicitUri to serve as the default implicit xmlns.
src/Controls/src/Xaml/XamlLoader.cs	Updates XmlNamespaceManager initialization to include the default implicit xmlns and additional prefixes.
src/Controls/src/SourceGen/CodeBehindGenerator.cs	Refactors methods (including ComputeXamlProjectItem and TryParseXaml) to support the new assembly caches and adjust type signatures accordingly.
src/Controls/src/Core/Properties/AssemblyInfo.cs	Adds an assembly attribute for the XAML 2009 namespace prefix.
src/Controls/src/Build.Tasks/*	Updates XAML parsing tasks to use module-specific xmlns prefix resolution.

Files not reviewed (3)

• Microsoft.Maui-vscode.sln: Language not supported
• src/Controls/src/SourceGen/Controls.SourceGen.csproj: Language not supported
• src/Controls/tests/Xaml.UnitTests/GlobalXmlns.xaml: Language not supported

Comments suppressed due to low confidence (1)

src/Controls/src/SourceGen/CodeBehindGenerator.cs:142

• The variable 'text' is not defined in this scope. It appears that the content from the ProjectItem should be used instead, for example by referencing projectItem.Text.

using var reader = XmlReader.Create(new StringReader(text.ToString(),