Support hosted services in WebAssemblyHost

[Copilot]

Fixes #33537

This PR adds support for hosted services (IHostedService) in WebAssemblyHost, enabling automatic startup and shutdown of background services in Blazor WebAssembly applications.

Problem

Applications using WebAssemblyHost (e.g. Blazor WebAssembly clients) did not support hosted services out of the box. Services registered with AddHostedService were ignored when using WebAssemblyHostBuilder, unlike in applications built with WebApplicationBuilder or HostApplicationBuilder. This prevented libraries like OpenTelemetry from working properly in Blazor WebAssembly apps, as they rely on hosted services for initialization.

Solution

The implementation adds hosted service lifecycle management to WebAssemblyHost:

• Service Discovery: Automatically discovers IHostedService implementations from the DI container
• Startup Integration: Starts hosted services during RunAsyncCore() after component state restoration
• Cleanup Integration: Stops hosted services during DisposeAsync() with proper error handling
• Minimal Footprint: Only ~40 lines of new code with no breaking changes

Usage

Developers can now register hosted services in Blazor WebAssembly apps:

var builder = WebAssemblyHostBuilder.CreateDefault(args);

// Register hosted services - they'll start automatically
builder.Services.AddSingleton<IHostedService, MyBackgroundService>();

var host = builder.Build();
await host.RunAsync(); // Hosted services start here

Implementation Details

• Added Microsoft.Extensions.Hosting.Abstractions reference to enable IHostedService support
• Hosted services start after component state restoration (matching the timing suggested in the original issue)
• Error handling ensures host disposal doesn't fail if hosted services throw exceptions during shutdown
• Performance optimized - only processes hosted services if any are registered

Testing

Added comprehensive test coverage:

• Service startup during host run
• Service shutdown during host disposal
• Error handling for faulty services
• Integration with service registration patterns

All existing WebAssemblyHost functionality remains unchanged and all tests pass.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support hosted services in WebAssemblyHost</issue_title>
<issue_description>### Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe the problem.

Applications using WebAssemblyHost (e.g. Blazor WebAssembly clients) do not support hosted services out of the box. Services registered with AddHostedService are ignored when using WebAssemblyHostBuilder, unlike in applications built with WebApplicationBuilder or HostApplicationBuilder.

Describe the solution you'd like

WebAssemblyHost should support automatically running hosted services registered with AddHostedService.

Additional context

This feature request has already been rejected in #33537. However, we might want to reconsider the decision. For example, support for hosted services might be beneficial for Open Telemetry integration in Blazor Wasm apps.</issue_description>

<agent_instructions>Look at

aspnetcore/src/Hosting/Hosting/src/Internal/HostedServiceExecutor.cs

Line 10 in 063ba34

private readonly IEnumerable<IHostedService> _services;

and

aspnetcore/src/Hosting/Hosting/src/Internal/WebHost.cs

Line 30 in 063ba34

private HostedServiceExecutor? _hostedServiceExecutor;

We want WebAssemblyHost to start the services here https://github.com/dotnet/aspnetcore/blob/main/src/Components/WebAssembly/WebAssembly/src/Hosting/WebAssemblyHost.cs#L139</agent_instructions>

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

@pavelsavara We should measure the size/perf impact of adding it by default for WASM

Fixes #63637

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