Enable MSBuildTask0004 task-authoring analyzer at error severity

[baronfel]

Summary

Re-enables MSBuildTask0004 (Microsoft.Build.TaskAuthoring.Analyzer) at its natural error severity, continuing the incremental "ratcheting" of the MSBuild thread-safe task-authoring analyzer started in #55053 (which enabled MSBuildTask0001).

What the rule is

MSBuildTask0004 — "API may cause issues in multithreaded tasks" (Warning by default). It flags APIs that can cause assembly version conflicts in a shared/multithreaded task host:

• Assembly.Load / LoadFrom / LoadFile / LoadWithPartialName
• Activator.CreateInstance(string, string) / Activator.CreateInstanceFrom
• AppDomain.Load / CreateInstance / CreateInstanceFrom

Why it can be enabled now

Unlike MSBuildTask0002/0003, the concerns of 0004 do not require the IMultiThreadableTask / TaskEnvironment migration — they are about avoiding problematic reflection/assembly-loading APIs, which can be enforced today. MSBuildTask0001/0002/0003/0005 remain at suggestion pending that migration (0002/0003 need TaskEnvironment; 0005 is transitive on the others).

The codebase already complies

There are zero direct MSBuildTask0004 violations. Verified by building all 12 IsMSBuildTaskProject projects with the repo-local SDK at error severity — every project reported Build succeeded (and because the repo builds warn-as-error, any violation would have broken the build):

• Microsoft.NET.Build.Containers
• Microsoft.DotNet.GenAPI.Task
• Microsoft.NET.Sdk.Publish.Tasks
• Microsoft.DotNet.ApiCompat.Task
• Microsoft.NET.Sdk.StaticWebAssets.Tasks
• Microsoft.NET.Sdk.Razor.Tasks
• Microsoft.TemplateEngine.Authoring.Tasks
• Microsoft.NET.Build.Tasks
• Microsoft.NET.Build.Extensions.Tasks
• Microsoft.NET.Sdk.BlazorWebAssembly.Tasks
• DotNetWatchTasks
• sdk-tasks

The rule was also confirmed active by temporarily injecting Assembly.LoadFrom("x") into a real ITask.Execute() and observing error MSBuildTask0004 fire (then reverted).

Changes

Only the two .editorconfig files (repo-root and src/StaticWebAssetsSdk/ which sets root = true and keeps its own in-sync copy): MSBuildTask0004 suggestion → error, with updated comments. No source fixes were needed.

[Copilot]

Pull request overview

This PR ratchets up enforcement of the MSBuild thread-safe task-authoring analyzer by promoting MSBuildTask0004 (“API may cause issues in multithreaded tasks”) from suggestion to error, ensuring banned assembly-loading/reflection APIs are blocked in MSBuild task projects.

Changes:

• Set dotnet_diagnostic.MSBuildTask0004.severity = error in the repo-root .editorconfig.
• Mirror the same severity update in src/StaticWebAssetsSdk/.editorconfig (which is root = true), and update the explanatory comments in both locations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.editorconfig	Promotes MSBuildTask0004 to error and updates ratcheting guidance/comments.
src/StaticWebAssetsSdk/.editorconfig	Keeps the duplicated analyzer severity block in sync (root-scoped editorconfig) and updates comments accordingly.