Add model_to_contextsize static method to the AzureChatOpenAI class consistent with the AzureOpenAI class

[RERobbins]

It's useful to get the contextsize for a model from class instances.

At the moment, I am working with both AzureChatOpenAI instances and AzureOpenAI instances.

Today I discovered the model_to_contextsize method in the AzureOpenAI class and was wondering why the same method isn't part of the AzureChatOpenAI class.

It would be useful for all completion and chat model classes.