fix(http-server-csharp): prevent _2 suffix on generated class/interface names from library namespace collisions

[Copilot]

When a library namespace (e.g. Azure.ResourceManager) defines interfaces or models with the same name as those in the user's @service namespace, the emitter was collecting both, causing alloy-js to rename the second occurrence with a _2 suffix — producing invalid C# where the class name and constructor name no longer match:

// Before fix — class and constructor names mismatch
public partial class OperationsController_2 : ControllerBase
{
    public OperationsController(IOperations_2 operations) { ... }
}

public interface IOperations_2 { }

public partial class ErrorResponse_2 : HttpServiceException
{
    public ErrorResponse(ErrorDetail error = default) { ... }
}

Root cause

getServiceInterfaces and getServiceModels traversed all non-standard namespaces in the global namespace tree, including library namespaces like Azure.ResourceManager. ARM RP specs define concrete Operations interfaces and common types (e.g. ErrorResponse) in those namespaces; when the same name appeared in the user's service namespace, both were emitted and the second got _2 appended to its class declaration.

Changes

• service-discovery.ts — getServiceInterfaces: Now calls listServices(program) and only collects interfaces from @service-decorated namespaces. Falls back to the previous all-namespace scan when no @service decorator is present (e.g. unit tests).

• service-resolution.ts — getServiceModels: Same scoping applied to the initial namespace model collection and the discoverReferencedModels starting points. Models transitively referenced by service operations (regardless of which namespace they live in) continue to be discovered and emitted via discoverModelsInType.

• test/service-discovery.test.ts: New regression tests covering both the interface/controller case and the model case.

• Fixes [Bug]: http-server-csharp: Some generated classes have wrong name, include _2 #11493

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commit: 29baac6

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• ✅ @typespec/http-server-csharp

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@typespec/http-server-csharp - fix ✏️

Fix generated class/interface names incorrectly receiving a _2 suffix (e.g. OperationsController_2, IOperations_2, ErrorResponse_2) when a non-service library namespace (such as Azure.ResourceManager) defines types with the same name as types in the @service namespace. The emitter now restricts its collection of interfaces and models to @service-decorated namespaces, preventing the naming collision.

[timotheeguerin]

Need to tweak a little the test and actual goal of the PR(comments are targetted for copilot to apply if they sound a little harsh)

[timotheeguerin]

Simplify this test, the azure mention are irrelevant, here we just want to test that we don't include unreachable types that are not in the service namespace
Do not include extra info keep the bare minimum needed in this test

[timotheeguerin]

those expect feel very speciifc to this instance and could easily not be testing the actual problem if the dedup logic change.
We should maybe simplify that to just check the whole set of generated models and make sure there is no unexpected one

[timotheeguerin]

lets reformulate this as we do not want to include type that are not defined in the service namespace unless they are referenced.