chore: Add support for derivedClasses metadata when implements interfaces

[filzrev]

This PR intended to fix a part of issue that is reported at #2531

Currently docfx generate derivedClasses metadata when inherited from class.
But derivedClasses metadata is not outputted for interface implements.

This PR add derivedClasses support for interfaces.

Additionally, Replace dictionary lookup logics to use CollectionsMarshal.GetValueRefOrAddDefault
to reduce dictionary double-lookup costs.

Test
I've manually confirmed derivedClasses metadata generation with following sample code.
And confirmed metadata is generated as expected.

namespace BuildFromProject;

public interface ISample{}

public abstract class SampleClassBase :ISample {}

public class Sample01 :ISample
{
    public int AAA{get;set;}
}

public class Sample02: ISample
{
    public int BBB{get;set;}
}

public class Sample03: SampleClassBase
{
    public int BBB{get;set;}
}

TODO
Currently derivedClasses metadata information is not rendered for interface HTML page.
It need to modify docfx templates also (by another PR)

[Copilot]

Pull Request Overview

This PR adds support for derivedClasses metadata when types implement interfaces and optimizes dictionary lookups using CollectionsMarshal.GetValueRefOrAddDefault.

• Introduces derivedClasses entries in interface snapshot JSON files
• Extends UpdateDerivedClassMapping to register classes under implemented interfaces
• Refactors dictionary additions to use GetValueRefOrAddDefault and updates AppendDerivedClass to include interfaces

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/docfx.Snapshot.Tests/SamplesTest.Seed/api/CatLibrary.IAnimal.html.view.verified.json	Added derivedClasses array for IAnimal
test/docfx.Snapshot.Tests/SamplesTest.Seed/api/BuildFromProject.Inheritdoc.Issue9736.IJsonApiOptions.html.view.verified.json	Added derivedClasses array for IJsonApiOptions
test/docfx.Snapshot.Tests/SamplesTest.Seed/api/BuildFromProject.Class1.IIssue8948.html.view.verified.json	Added derivedClasses array for IIssue8948
test/docfx.Snapshot.Tests/SamplesTest.CSharp/api/CSharp11.StaticAbstractMembersInInterfaces.IGetNext-1.html.view.verified.json	Added derivedClasses array for IGetNext
src/Docfx.Dotnet/ManagedReference/Resolvers/SetDerivedClass.cs	Added interface handling in UpdateDerivedClassMapping, refactored dictionary logic, and updated AppendDerivedClass to include interfaces

Comments suppressed due to low confidence (2)

src/Docfx.Dotnet/ManagedReference/Resolvers/SetDerivedClass.cs:52

• [nitpick] The variable name superClass is ambiguous in the context of interfaces. Rename it to something like interfaceName to clarify its purpose.

var superClass = implements[implements.Count - 1];

src/Docfx.Dotnet/ManagedReference/Resolvers/SetDerivedClass.cs:23

• No automated tests were added to verify derivedClasses generation for interfaces. Consider adding unit tests covering classes that implement one or multiple interfaces to ensure correct metadata output.

private void UpdateDerivedClassMapping(List<MetadataItem> items, Dictionary<string, List<string>> _derivedClassMapping)