[StaticWebAssets] Move static web assets tests into their own project

[javiercn]

• I asked copilot to move the StaticWebAssets tests to their own project since they were on the .Razor.Tests project
• Confirmation of tests running on CI
  

[Copilot]

Pull Request Overview

This PR moves the Static Web Assets tests out of the Razor test project into their own dedicated test project and updates all related namespaces, references, and resources.

• Created a new Microsoft.NET.Sdk.StaticWebAssets.Tests project with its own .csproj, embedded resources, and targets file.
• Updated namespaces and manifest resource paths in existing test files to point to the new project.
• Adjusted solution, project references, and added InternalsVisibleTo to wire up the new test project.

Reviewed Changes

Copilot reviewed 50 out of 155 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/content/ExternalStaticAssets.targets	New MSBuild targets file defining external static assets logic
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssetsIntegrationTest.cs	Refactored namespace, manifest stream path, and SDK handling
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/Microsoft.NET.Sdk.StaticWebAssets.Tests.csproj	New test project setup, dependencies, and embedded resources
src/StaticWebAssetsSdk/Tasks/Microsoft.NET.Sdk.StaticWebAssets.Tasks.csproj	Added InternalsVisibleTo for the new test project
sdk.slnx	Included the new test project in the solution

Comments suppressed due to low confidence (4)

test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssetsFingerprintingTest.cs:12

• The file name (StaticWebAssetsFingerprintingTest.cs) does not match the class name (StaticWebAssetsContentFingerprintingIntegrationTest). Consider renaming the file to StaticWebAssetsContentFingerprintingIntegrationTest.cs or adjusting the class name to align with the file.

public class StaticWebAssetsContentFingerprintingIntegrationTest(ITestOutputHelper log) : AspNetSdkBaselineTest(log)

test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/ApplyCssScopesTest.cs:11

• The file is named ApplyCssScopesTest.cs but the class is named ApplyAllCssScopesTest. Rename one or the other to keep file and class names consistent.

    public class ApplyAllCssScopesTest

test/Microsoft.NET.Sdk.StaticWebAssets.Tests/ScopedCssIntegrationTests.cs:11

• The file is named ScopedCssIntegrationTests.cs (plural) but the class is ScopedCssIntegrationTest (singular). Align the file name and class name for clarity.

    public class ScopedCssIntegrationTest(ITestOutputHelper log)

test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssetsIntegrationTest.cs:478

• Add a test case to cover the branch where the project file has no Sdk attribute, ensuring the null branch and attribute insertion logic are exercised.

                        if (sdkAttribute == null)