Migrate ComputeReferenceStaticWebAssetItems

[AlesProkop]

Fixes dotnet/msbuild#14055

Context

Migrates ComputeReferenceStaticWebAssetItems to support MSBuild multithreaded task execution by avoiding process-CWD-based path resolution.

Changes Made

• Added [MSBuildMultiThreadableTask] to ComputeReferenceStaticWebAssetItems.
• Implemented IMultiThreadableTask and added a TaskEnvironment property with TaskEnvironment.Fallback default.
• Updated ComputeReferenceStaticWebAssetItems to pass TaskEnvironment when loading static web assets.
• Added a TaskEnvironment overload for StaticWebAsset.AssetsByTargetPath(...), preserving the existing overload via TaskEnvironment.Fallback.
• Updated MakeReferencedAssetOriginalItemSpecAbsolute path resolution to use TaskEnvironment.GetAbsolutePath(...) before Path.GetFullPath(...), preserving canonicalization while resolving relative paths against the project directory.
• Added a focused regression test verifying OriginalItemSpec is absolutized against the injected project directory rather than the process current directory.

Testing

• Built src\StaticWebAssetsSdk\Tasks\Microsoft.NET.Sdk.StaticWebAssets.Tasks.csproj successfully.
• Built test\Microsoft.NET.Sdk.StaticWebAssets.Tests\Microsoft.NET.Sdk.StaticWebAssets.Tests.csproj for net11.0 successfully.
• Ran Microsoft.NET.Sdk.StaticWebAssets.Tests.ComputeReferenceStaticWebAssetItemsTest successfully.

[Copilot]

Pull request overview

This PR migrates ComputeReferenceStaticWebAssetItems to be safe for MSBuild multi-threaded task execution by eliminating reliance on process current-directory (CWD) for path resolution, aligning with the TaskEnvironment-based execution model.

Changes:

• Marked ComputeReferenceStaticWebAssetItems as [MSBuildMultiThreadableTask], implemented IMultiThreadableTask, and threaded TaskEnvironment into asset loading and path absolutization.
• Added a TaskEnvironment overload to StaticWebAsset.AssetsByTargetPath(...) and updated normalization to canonicalize via TaskEnvironment-based absolute paths.
• Added a regression test ensuring OriginalItemSpec is resolved against the injected project directory (not the process CWD).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/ComputeReferenceStaticWebAssetItemsTest.cs	Adds regression coverage for OriginalItemSpec absolutization using an injected TaskEnvironment project directory.
src/StaticWebAssetsSdk/Tasks/Data/StaticWebAsset.cs	Introduces AssetsByTargetPath(..., TaskEnvironment) overload and ensures item hydration/normalization can be rooted in TaskEnvironment.
src/StaticWebAssetsSdk/Tasks/ComputeReferenceStaticWebAssetItems.cs	Makes the task multi-threadable and uses TaskEnvironment for MT-safe asset parsing and OriginalItemSpec absolutization.

[AlesProkop]

Expert MSBuild multithreaded task migration review: no high-confidence actionable issues found.

Checked:

• ComputeReferenceStaticWebAssetItems is marked [MSBuildMultiThreadableTask], implements IMultiThreadableTask, and defaults TaskEnvironment to TaskEnvironment.Fallback.
• The task threads TaskEnvironment into static web asset hydration, so relative path-bearing metadata resolves against the project directory rather than process CWD.
• The migrated OriginalItemSpec, ContentRoot, and RelatedAsset paths keep the previous canonicalized full-path behavior via FileInfo/DirectoryInfo.
• No unintended AbsolutePath leakage into logs or output values was found, and no unsafe process CWD/environment/process-start APIs were introduced.

[jankratochvilcz]

Thanks, one bigger question here :-)

[OvesN]

LGTM