Add SelectRuntimeIdentifierSpecificItems MSBuild task for filtering items by compatible RuntimeIdentifier

[Copilot]

Fixes #50614

• Implement SelectRuntimeIdentifierSpecificItems MSBuild task
• Add comprehensive unit tests for the task
• Add UsingTask declaration to make task ambiently available to downstream consumers
• Remove #nullable disable directive and add proper nullable reference type annotations
• Simplify runtime compatibility check to use runtimeGraph.AreCompatible() directly
• Update nullable annotations for Required properties to use null! pattern

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@baronfel note: I've reviewed and tested this PR and am happy with its semantics and coverage.

[Copilot]

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[Copilot]

Pull Request Overview

This PR introduces a new MSBuild task SelectRuntimeIdentifierSpecificItems that filters items based on Runtime Identifier compatibility using NuGet's runtime graph. The task enables downstream consumers to filter assets by RID compatibility rather than just equality, supporting container incrementality scenarios.

• Implements the core task with proper nullable reference type annotations
• Adds comprehensive unit tests covering compatibility filtering, exact matches, and edge cases
• Makes the task ambiently available through UsingTask declaration in targets file

Reviewed Changes

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

File	Description
Microsoft.NET.RuntimeIdentifierInference.targets	Adds UsingTask declaration to make the new task available to consumers
SelectRuntimeIdentifierSpecificItems.cs	Implements the main task logic with RID compatibility filtering using RuntimeGraph
GivenASelectRuntimeIdentifierSpecificItems.cs	Provides comprehensive unit test coverage for all task scenarios

[Copilot]

Properties marked with [Required] should not be nullable. The MSBuild framework expects required properties to have non-null values, making the nullable annotation misleading and potentially causing confusion for consumers.

Suggested change

	[Required]
	public string? RuntimeIdentifierGraphPath { get; set; }
	public string RuntimeIdentifierGraphPath { get; set; }

[baronfel]

/backport to release/10.0.1xx

[github-actions]

Started backporting to release/10.0.1xx: https://github.com/dotnet/sdk/actions/runs/17780051830