Migrate ApplyCompressionNegotiation to IMultiThreadableTask

[jankratochvilcz]

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

Fixes dotnet/msbuild#14031

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

Target	non-MT (ms)	MT (ms)	Δ (ms)	Δ (%)
ResolveBuildCompressedStaticWebAssets	325	6,423	+6,098	+1,876 %

Changes

• [MSBuildMultiThreadableTask] on the concrete class. No IMultiThreadableTask plumbing needed — the task body has no filesystem / environment / process I/O (pure metadata propagation and string composition).

Compatibility sins audit (per migration skill)

24/24 dimensions clean — attribute-only opt-in. No Path.* / File.* / Directory.* / Environment.* / Process.* calls; no [Output] AbsolutePath leakage; no new null-coalescing; no try/catch reorg; no dictionary keys requiring canonicalization; no exception-type divergence.

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 annotates ApplyCompressionNegotiation as an MSBuild multi-threadable task by adding [MSBuildMultiThreadableTask], with the goal of enabling MSBuild to schedule it concurrently and improve build performance in static web assets compression negotiation.

Changes:

• Add [MSBuildMultiThreadableTask] to ApplyCompressionNegotiation.

[OvesN]

Will be ok after we merge #54708

[OvesN]

ToAssetDictionary->fromTaskItem->normalize->getFullPath