Fixes to Assistant, VectorStore client methods

Fixes #62 -- the remaining convenience methods in AssistantClient and VectorStoreClient have optional CancellationTokens added as method parameters, including the typed instance convenience overloads
Closes #61 -- adds missing [EditorBrowsable(EditorBrowsableState.Never)] attributes to AssistantClient protocol methods, improving strongly typed convenience method discoverability
Addresses a yet-unreported problem with the new max_num_results property for file_search not being serialized/deserialized

openai / openai-dotnet