.Net: Bug Agents Assistants V2: Update usage of `CancellationToken` once Open AI SDK version is updated

[crickman]

Several SDK methods in the Open AI AssistantsClient are missing CancellationToken in the method signature for the beta-2 package. These fixes have been realized and are present in the beta-7 package.

Since our usage of the Open AI SDK is bound to the Azure Open AI SDK (currently targeting Open AI beta-2), we are limited in our ability to uplevel the package of the Open AI SDK.

TODO: Update usage in the SK Agent Framework once new packages are available.

[crickman]

Addressed in #7151