Auto-determine the anthropic model context window size

[bhouston]

There is support within the Anthropic SDK for getting the model context window size. Right now the anthropic LLM provider has all the model sizes hard coded. Here is an example of how it can be determined:

    const response = await this.anthropicClient.models.list();
      const model = response.data.find(m => m.id === modelName);
      
      if (!model) {
        return null;
      }

      return {
        name: model.id,
        contextWindow: model.context_window,
        provider: 'anthropic',
      };

We should replace the existing hard coded method with this proper detection method.

[bhouston]

/mycoder, can you implement this improvement and make a PR?

[bhouston]

This is implemented.