Migrate ApplyCssScopes to IMultiThreadableTask

[jankratochvilcz]

Migrates ApplyCssScopes (src/StaticWebAssetsSdk/Tasks/ScopedCss/ApplyCssScopes.cs) to multi-threadable by adding [MSBuildMultiThreadableTask].

Fixes dotnet/msbuild#14033

Perfstar regression (14d, Windows, paired MT vs non-MT, p50 sum across iterations)

Target	non-MT (ms)	MT (ms)	Δ (ms)	Δ (%)
ResolveCssScopes	116	5,134	+5,018	+4,326 %

Changes

• [MSBuildMultiThreadableTask] on the concrete class. No IMultiThreadableTask plumbing needed — the task body matches scoped CSS items to inputs purely in memory; Environment.NewLine is the only Environment.* reference and is a string constant.

Compatibility sins audit (per migration skill)

24/24 dimensions clean — attribute-only opt-in.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[jankratochvilcz]

No decoy-style unit test added — not viable here

This is an attribute-only opt-in. A decoy-CWD test of the form used for GenerateStaticWebAssetsManifest / MergeConfigurationProperties is not meaningful: the task body contains no File.*, Directory.*, Path.GetFullPath, Environment.* (except Environment.NewLine, a string constant), or Process.* calls whose behavior could diverge under a different process CWD. There is therefore no observable runtime behavior the migration could regress for the decoy test to catch — the attribute purely toggles whether MSBuild wraps the task in a TaskHost, which a unit test cannot exercise.

[Copilot]

Pull request overview

This PR opts ApplyCssScopes into MSBuild’s multi-threadable task execution by annotating it with [MSBuildMultiThreadableTask], aligning it with MSBuild’s parallel task execution requirements and addressing the regression described in msbuild#14033.

Changes:

• Add [MSBuildMultiThreadableTask] to ApplyCssScopes.

[jankratochvilcz]

/azp run dotnet-sdk-public-ci

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[jankratochvilcz]

/azp run dotnet-sdk-public-ci

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[jankratochvilcz]

/azp run dotnet-sdk-public-ci

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[jankratochvilcz]

/azp run dotnet-sdk-public-ci

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