Fix checkout ordering response handling

[cursor]

Summary

• Ensure checkout order creation disposes the HTTP response and throws on non-success ordering API responses.
• Add WebApp unit coverage for request ID header behavior and non-success response handling.
• Add a Playwright checkout regression that logs in, adds an item, places an order, and verifies redirect to /user/orders.

Runtime Evidence

• Pre-fix instrumentation showed a simulated ordering response with statusCode:500,isSuccess:false returned without throwing; the new unit test failed with Expected HttpRequestException but no exception was thrown.
• Post-fix instrumentation showed the same 500 response followed by exceptionType: HttpRequestException.
• Post-fix checkout E2E logs showed the real flow reached Ordering.API, mediator returned true, HTTP returned 200, and basket deletion completed.

Verification

• dotnet restore eShop.Web.slnf -p:NuGetAudit=false --ignore-failed-sources && dotnet build eShop.Web.slnf -p:NuGetAudit=false -p:LibraryRestore=False --no-restore
• dotnet test --project tests/WebApp.UnitTests/WebApp.UnitTests.csproj --no-restore
• ESHOP_USE_HTTP_ENDPOINTS=1 USERNAME1=bob PASSWORD='Pass123$' npx playwright test --project='e2e tests logged in' e2e/CheckoutTest.spec.ts --reporter=line

Note: restore/build/test emitted known NU1801 warnings from intermittent api.nuget.org connectivity, but completed successfully.