Document HostingStartup incompatibility with .NET Aspire integrations

[Copilot]

This PR addresses the issue where users are unable to configure .NET Aspire integrations when using HostingStartup because Aspire integrations require IHostApplicationBuilder instead of IWebHostBuilder.

Changes Made

New Troubleshooting Documentation

Added comprehensive documentation at docs/troubleshooting/hosting-startup-not-supported.md that:

• Explains the incompatibility: HostingStartup only provides access to IWebHostBuilder, while .NET Aspire integrations require IHostApplicationBuilder
• Documents symptoms: Compilation errors, runtime configuration issues, and missing telemetry/resilience features
• Provides migration strategies: Four different approaches with complete code examples:
  1. Direct use of IHostApplicationBuilder (recommended)
  2. Creating configuration extensions
  3. Feature flags/configuration-based registration
  4. Dependency injection for plugin architecture

Updated Integration Documentation

Enhanced docs/fundamentals/integrations-overview.md with an important notice highlighting the HostingStartup limitation and linking to the migration guidance.

Table of Contents

Updated docs/toc.yml to include the new troubleshooting document in the appropriate section.

Example Migration Pattern

Before (HostingStartup - doesn't work):

public class MyDatabaseStartup : IHostingStartup
{
    public void Configure(IWebHostBuilder builder)
    {
        // This won't work with Aspire integrations
        builder.ConfigureServices(services => { ... });
    }
}

After (IHostApplicationBuilder - works with Aspire):

var builder = WebApplication.CreateBuilder(args);
builder.AddServiceDefaults();
builder.AddNpgsqlDbContext<MyDbContext>("postgres");

This documentation helps users maintain modularity while leveraging .NET Aspire's modern hosting patterns, observability features, and service discovery capabilities.

Fixes #3861.

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/integrations-overview.md	docs/fundamentals/integrations-overview
docs/toc.yml	docs/toc
docs/troubleshooting/hosting-startup-not-supported.md	docs/troubleshooting/hosting-startup-not-supported