Forward the cancellation token in the synchronous ModifyAssistant

[adityasingh2400]

The synchronous AssistantClient.ModifyAssistant overload passed a literal null to the protocol method instead of the caller's cancellation token, so the token was dropped before the request was sent and the call could not be cancelled. Every other convenience method on this client already forwards its token through ToRequestOptions, including ModifyAssistant's own asynchronous twin one method above it, so this was an isolated gap rather than a deliberate difference. The change makes the synchronous overload match its sibling and honor the behavior its own documentation comment describes. Callers who pass a default token see no change at all, because ToRequestOptions returns null for default, which is exactly what was being passed before. The added mock transport regression test exercises both overloads and fails only on the synchronous one without this fix.

[adityasingh2400]

Good catch, that was a real gap. ExpectSyncPipeline = !IsAsync set the transport up for both modes, but the body only ever called ModifyAssistantAsync, so the synchronous path this PR fixes was never reached and the sync half of the fixture proved nothing.

Now branched on IsAsync, matching StreamingRunSurfacesErrorEventAsException:

if (IsAsync)
{
    Assert.That(
        async () => await client.ModifyAssistantAsync("asst_abc", new AssistantModificationOptions(), cancellationSource.Token),
        Throws.InstanceOf<OperationCanceledException>());
}
else
{
    Assert.That(
        () => client.ModifyAssistant("asst_abc", new AssistantModificationOptions(), cancellationSource.Token),
        Throws.InstanceOf<OperationCanceledException>());
}

One thing I should be straight about: I could not run this locally. global.json pins SDK 10.0.400 and I only have 9.0.301, so I am relying on CI rather than a local green run. The sync overload takes CancellationToken cancellationToken = default at AssistantClient.cs:201, so the call shape matches, but please treat the pass as unverified by me until CI says otherwise.