StaticWebAssets FUTDC: track endpoints manifest as input/output for referenced projects

[Copilot]

• Add EndpointsBuildManifestPath metadata to GetStaticWebAssetsProjectConfiguration
• Write endpoints manifest path to tracking file during real builds
• Add endpoints manifest as UpToDateCheckOutput
• Fix timing bug: add design-time path to ResolveReferencedProjectsStaticWebAssetsConfiguration that queries referenced projects' manifest paths without the BuildReference=true filter (safe because GetStaticWebAssetsProjectConfiguration only reads properties)
• Guard against duplicates: design-time path only runs when _ReferencedProjectsConfiguration is empty (i.e., when real-build path couldn't run)
• Add ResolveReferencedProjectsStaticWebAssetsConfiguration to CollectUpToDateCheckInputDesignTimeDependsOn chain so VS triggers the design-time path on initial project load
• Add CollectUpToDateCheckInputOutputsDesignTime_IncludesReferencedProjectsManifests_OnInitialLoad test that verifies referenced manifests are tracked without a prior build
• All 4 design-time FUTDC tests pass

Original prompt

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This section details on the original issue you should resolve

<issue_title>StaticWebAssets FUTDC does not track referenced project static web asset changes</issue_title>
<issue_description>## Summary

When a Blazor Web App with WebAssembly + Global interactivity has a CSS file modified in the Client project and the app is restarted from Visual Studio, the Server project's build is skipped by VS's Fast Up-to-Date Check (FUTDC). This leaves the Server's .staticwebassets.endpoints.json manifest stale with old fingerprints and ETags, causing the app to fail at runtime with "An unhandled error has occurred."

The issue was identified while investigating dotnet/aspnetcore#65738.

Reproduction

1. Create a new .NET 10 Blazor Web App with WebAssembly + Global interactivity in VS.
2. Run the project (Ctrl+F5 or F5).
3. Stop the app.
4. Modify MainLayout.razor.css in the Client project (e.g., change a background-color).
5. Run the project again (Ctrl+F5).
6. The page fails with "An unhandled error has occurred." Reload does not help.

Workaround: Right-click the Client project in Solution Explorer and select "Rebuild" before running.

Root cause

The SDK has an existing mechanism for tracking cross-project static web asset dependencies for FUTDC, but it has a timing bug that prevents it from working.

The existing mechanism

1. Build-time target ResolveReferencedProjectsStaticWebAssetsConfiguration (Microsoft.NET.Sdk.StaticWebAssets.References.targets) writes referenced project BuildManifestPath values to staticwebassets.references.upToDateCheck.txt.

2. Design-time target CollectStaticWebAssetInputsDesignTime (Microsoft.NET.Sdk.StaticWebAssets.Design.targets) reads this file and adds the paths as UpToDateCheckInput items in Set="StaticWebAssets".

The timing bug

ResolveReferencedProjectsStaticWebAssetsConfiguration is a build-time target. CollectStaticWebAssetInputsDesignTime is a design-time target hooked into CollectUpToDateCheckInputDesignTimeDependsOn.

On initial project load, VS runs the design-time target before any build has occurred. At that point, staticwebassets.references.upToDateCheck.txt does not exist yet. The Condition="Exists(...)" causes ReadLinesFromFile to be skipped, and VS CPS caches the empty _StaticWebAssetReferenceUpToDateCheckInput result. Subsequent full builds create the file, but the design-time cache is never invalidated because intermediate output file changes do not trigger design-time re-evaluation.

How this was established

1. VS FUTDC verbose logging confirmed the Server project's UpToDateCheckInput items in Set="StaticWebAssets" only contained Server-owned wwwroot files. No Client project static web asset outputs were listed.
2. The staticwebassets.references.upToDateCheck.txt file existed on disk and correctly contained the Client's staticwebassets.build.json path.
3. Direct ETag comparison confirmed the stale manifest: the Server bin had ETags from the initial build (fingerprint 69a47x8btv) while the Client had regenerated with a new fingerprint (w5s9wppyq9).
4. Manually adding <UpToDateCheckInput Include="...\staticwebassets.build.json" Set="StaticWebAssets" /> to the Server .csproj fixed the issue, confirming the root cause.</issue_description>

<agent_instructions>Make a proposal about adding the endpoints file to the fast up to date check. We already have additions like this on Static Web Assets SDK for other things</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes StaticWebAssets FUTDC does not track referenced project static web asset changes #53425

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[javiercn]

Should include also the developer manifest too, just in case.