Add lightweight Blazor SSR islands sample

[balajibashyam]

Summary

Adds a minimal .NET 10 companion sample for the Blazor SSR & Interactive Islands: Streaming Rendering, Auto Render Mode & Progressive Enhancement tutorial.

What's included

• New sample: blazor/ssr-interactive-islands/ — 16 files

  • Static SSR product catalog route with [StreamRendering]
  • Server-rendered dynamic metadata
  • Delayed reviews placeholder streamed in a second response chunk
  • One Interactive Server cart island (no global interactive mode)
  • JSON-serializable ProductSummary[] crossing the render-mode boundary
  • Add, increment, decrement, and remove cart interactions
  • Deterministic item counts and monetary totals
  • UseStatusCodePagesWithReExecute for custom 404
  • 7 bUnit component tests + 1 config test + 2 ASP.NET Core integration tests

• Root README: New sample row + updated Blazor tree (all three sample folders)

• GitHub Actions: New test-blazor-ssr-islands job; path filters updated

Deliberately omitted (stays in the full tutorial)

• Interactive WebAssembly, Interactive Auto, offline support, JavaScript interop, APIs, databases, cart persistence, Docker

Versions

• .NET SDK: 10.0.302
• Application NuGet packages: none
• bunit: 2.8.6
• Microsoft.AspNetCore.Mvc.Testing: 10.0.10
• xunit.v3: 3.2.2

Validation

• dotnet restore — passed
• dotnet build --configuration Release — passed (0 errors, 0 warnings)
• dotnet test --configuration Release — passed (10/10)
• HTTP GET / — 200 (all 3 products, blazor.web.js, review content)
• HTTP GET /this-page-does-not-exist — 404 (custom Not Found, no redirect)
• Streaming: loading placeholder at 0.000s, reviews at 0.696s (verified via Kestrel chunked response)
• Security scan — clean