Add csharp customizations for DomainServices migration

[cxznmhdcxz]

Adds C# management-plane (http-client-csharp-mgmt) customizations for onboarding the Azure.ResourceManager.DomainServices SDK (API version 2025-10-01-preview).

Changes

• tspconfig.yaml: add @azure-typespec/http-client-csharp-mgmt emitter section (namespace Azure.ResourceManager.DomainServices).
• client.tsp (new), C#-scoped decorators:
  • @@alternateType(Resource.id, armResourceIdentifier) - common-types v3 models id as a plain string; maps it to ResourceIdentifier so generated ResourceData ctors compile.
  • @@clientName(UnsuspendDomainServiceResponse -> DomainServiceUnsuspendResult) - fixes AZC0030 (model may not end in Response).
  • @@clientName(DomainServices -> DomainServiceOperationGroup) - the DomainServices interface generated an internal REST class colliding with the namespace's last segment; rename avoids the _DomainServices namespace escape (internal, no public API impact).

Companion SDK PR: Azure/azure-sdk-for-net onboarding Azure.ResourceManager.DomainServices.

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API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
TypeSpec	Microsoft.AAD
C#	Azure.ResourceManager.DomainServices
Java	com.azure.resourcemanager:azure-resourcemanager-domainservices

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