Add rate limit specific troubleshooting tips to Duo Chat Runbook

This update to the Duo Chat README adds information about rate limits for AI actions. It specifies a limit of 160 calls per 8 hours for each authenticated user, explaining how this limit is applied and what error is returned when exceeded. The document also provides instructions for checking current rate limit usage, resetting rate limits temporarily, and suggestions for handling persistent issues. These additions help users and developers understand and manage the rate limiting system for Duo Chat's AI features.