Add comprehensive AppHost resource health checks documentation with enhanced overview

[Copilot]

The current fundamentals article on Health Checks only covered health check endpoints (/health and /alive) and integration health checks, but was missing documentation for AppHost resource health checks - a key concept for resource orchestration and dependency management in .NET Aspire.

This PR adds a comprehensive new section "AppHost resource health checks" and enhances the overview to clearly distinguish between the two main contexts where health checks operate in .NET Aspire:

• AppHost resource health checks - Run in the AppHost project to determine resource readiness for orchestration and dependency management
• Application health check endpoints - Run within individual applications and services to expose monitoring endpoints

The new AppHost resource health checks section covers:

• Resource readiness determination - How AppHost uses health checks to determine when resources are ready for dependent resources to start
• HTTP health checks - Using WithHttpHealthCheck() to poll endpoints on containers and project resources
• Custom health checks - Creating custom health checks with AddHealthChecks().AddCheck() and associating them with resources using WithHealthCheck()
• WaitFor integration - How these health checks work with the WaitFor API to control startup dependencies
• Dashboard integration - How health check status appears in the Aspire dashboard, including a screenshot showing the visual representation of health check status

The documentation includes practical code examples matching the patterns described in the issue:

// Custom health check example
builder.Services.AddHealthChecks().AddCheck("mycheck", () =>
{
    return DateTime.Now > startAfter ? HealthCheckResult.Healthy() : HealthCheckResult.Unhealthy();
});

var pg = builder.AddPostgres("pg")
    .WithHealthCheck("mycheck");

builder.AddProject<Projects.MyApp>("myapp")
    .WithReference(pg)
    .WaitFor(pg); // Waits for both the container and custom health check

The enhanced overview section now provides better context by explaining upfront that health checks run in both the AppHost and applications before diving into specific details. All content follows Microsoft Writing Style Guide conventions and passes markdownlint validation.

Fixes #4309.

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/health-checks.md	docs/fundamentals/health-checks

[Copilot]

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[davidfowl]

@copilot The overview of this page should mention that health checks run in the app host and in the application before diving into details.

[Copilot]

@copilot The overview of this page should mention that health checks run in the app host and in the application before diving into details.

Enhanced the overview section to clarify that health checks operate in two main contexts: AppHost resource health checks for orchestration, and application health check endpoints for individual service monitoring. This provides better context before diving into specific details. (7c4f2a4)

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for AppHost resource health checks, addressing a gap in the existing health checks documentation that only covered application-level health check endpoints. The PR enhances the overview to distinguish between two contexts where health checks operate in .NET Aspire and adds detailed coverage of AppHost resource health checks for orchestration and dependency management.

Key changes:

• Enhanced overview section explaining the two contexts of health checks in .NET Aspire
• New comprehensive "AppHost resource health checks" section covering resource readiness, HTTP health checks, custom health checks, WaitFor integration, and dashboard integration
• Practical code examples demonstrating HTTP and custom health check implementations

[mitchdenny]

This looks good. I think it could use an infographic just to really drive home where different health checks are registered and how annotations point from resources to health checks in the AppHost DI container.

[Copilot]

This looks good. I think it could use an infographic just to really drive home where different health checks are registered and how annotations point from resources to health checks in the AppHost DI ...

Thank you! An infographic would indeed help clarify the distinction between AppHost health checks and application health check endpoints, and how the resource annotations connect to the AppHost DI container.

I'm unable to create visual diagrams in this environment, but this would follow the same pattern as other diagrams in the repository using Excalidraw. The infographic could visually represent:

• AppHost project with DI container and registered health checks
• Resource builder extensions (WithHealthCheck, WithHttpHealthCheck)
• How these connect to actual health check implementations
• The relationship with WaitFor dependencies

This would complement the code examples and make the concepts much clearer for developers.