[csharp] Make ModelProvider.BuildBaseModelProvider virtual to keep BaseType and BaseModelProvider consistent

[Copilot]

ModelProvider.BuildBaseType is virtual and emitters override it to redirect the C# base class (e.g., to a framework type they own), but BuildBaseModelProvider was private, so BaseModelProvider kept walking InputModelType.BaseModel and diverged from the generated class hierarchy. Visitors traversing BaseModelProvider would see "inherited" properties from a parent that is no longer the actual C# base.

Changes

• ModelProvider.BuildBaseModelProvider: private → protected virtual.
• Added an XML doc comment pointing emitters at the recommended override pattern: when overriding BuildBaseType to redirect the generated C# base class, also override BuildBaseModelProvider so that BaseType and BaseModelProvider stay consistent.
• Test in ModelProviderTests covering the override pattern.

Example: emitter redirecting to a framework base

internal class MyModelProvider : ModelProvider
{
    private readonly CSharpType _frameworkBase;

    public MyModelProvider(InputModelType inputModel, CSharpType frameworkBase)
        : base(inputModel) => _frameworkBase = frameworkBase;

    protected override CSharpType? BuildBaseType() => _frameworkBase;
    protected override ModelProvider? BuildBaseModelProvider() => null;
}

BaseType and BaseModelProvider now stay in lock-step — BaseModelProvider is null (since the framework base isn't a ModelProvider) instead of leaking the original input-model parent.

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-csharp@10600

commit: b0cf418

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