Add support for azure managed identity authentication and convert MCP generation to llitellm instead of OpenRouter

[jgbradley1]

This PR swaps out the use of OpenRouter in the MCP generation code to LiteLLM instead which is more in line with other parts of the codebase (RULER) and doesn’t require the creation of accounts on another website like OpenRouter.

Some minor bug fixes are addressed too:

• modified generate_scenarios() to always guarantee the correct number of scenarios is generated instead of relying on the LLM.
• Users can leverage litellm documentation to set environment variables for base url and api keys instead of ART-specific env variables.
• If an Azure OpenAI endpoint is used, support for managed identity authentication has been added for organizations that redirect/disable the use of API keys.