[http-client-csharp] Emit async modifier when customizing an async method via a partial method (CS4032)

[Copilot]

Customizing an async client method via a C# partial method declaration produced an implementing declaration without the async modifier, while the generated body uses await — resulting in compiler error CS4032.

A partial declaration cannot carry async (that modifier belongs to the implementing declaration), so building the implementation signature from the customer's declaration modifiers dropped it. The generated (non-custom) path adds Async separately, so only the custom-code path was affected.

Changes

• PartialMethodCustomization.BuildPartialSignature: added an optional additionalModifiers parameter, OR-ed into the resulting signature modifiers. Backward-compatible and reusable by downstream emitters.
• ScmMethodProviderCollection: both custom-code call sites (BuildProtocolMethod, BuildConvenienceMethod) now pass MethodSignatureModifiers.Async when isAsync && _pagingServiceMethod == null, mirroring the condition already used by the generated path.
• Tests: added CanCustomizeAsyncMethodSignature and its custom-code test data asserting the Async modifier is present on the customized async protocol method.

Result

Given a custom partial declaration:

public partial Task<ClientResult> GetWidgetAsync(string id, RequestOptions options);

the generated implementation now emits:

private async partial Task<ClientResult> GetWidgetAsync(string id, RequestOptions options)
{
    using PipelineMessage message = CreateGetWidgetRequest(id, options);
    return ClientResult.FromResponse(await Pipeline.ProcessMessageAsync(message, options).ConfigureAwait(false));
}

Note: @typespec/http-client-csharp is not registered in Chronus, so no changelog entry was added.

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