Fix design-time services documentation for EF Core 6.0+

[Copilot]

Fixes #3390 and #4013

The design-time services documentation was outdated and contained broken code samples that don't work with EF Core 6.0+. This PR updates the documentation to reflect the current state of the design-time services API.

Issues Fixed

CustomTools.cs sample broken in .NET 6.0+ (#4013)

The main issue was that serviceProvider.GetService<IMigrationsScaffolder>() returns null in EF Core 6.0+ because the service registration pattern had changed. The sample now uses:

// Create design-time services
var serviceCollection = new ServiceCollection();

// Add the core design-time services
serviceCollection.AddEntityFrameworkDesignTimeServices();

// Add services from the DbContext
serviceCollection.AddDbContextDesignTimeServices(db);

var serviceProvider = serviceCollection.BuildServiceProvider();

// Use GetRequiredService instead of GetService for better error handling
var migrationsScaffolder = serviceProvider.GetRequiredService<IMigrationsScaffolder>();

Outdated service list (#3390)

The documentation listed only 10 design-time services when there are now 23+ services available. Added all current services including:

• ICandidateNamingService - Generates candidate names for entities and properties
• ICSharpMigrationOperationGenerator - Generates C# code for migration operations
• ICompiledModelCodeGenerator - Generates code for compiled model metadata
• IPrecompiledQueryCodeGenerator - Generates code for precompiled queries
• And many others

Additional Improvements

• Updated PackageReference example from version 3.1.9 to 8.0.0
• Added advanced customization example showing how to override CSharpHelper.ShouldUseFullName
• Added explanatory note about the streamlined service registration in EF Core 6.0+
• Fixed grammar in tools.md ("paths" instead of "path")
• Validated changes by creating and building a test project to ensure the sample code works

The updated documentation now accurately reflects the current EF Core design-time services API and provides working examples for developers.

Warning

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Original prompt

Fix @dotnet/EntityFramework.Docs/issues/3390 and @dotnet/EntityFramework.Docs/issues/4013 by updating the documentation for design services. Read all the comments to make sure you have enough context. Also, examine the implementation of @dotnet/efcore/files/src/EFCore.Design/Design/Internal/DesignTimeServicesBuilder.cs

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