Unified message history APIs for simpler integrations

We removed get_messages_for_session and get_messages_from_last_n_runs in favor of get_messages, get_session_messages, and get_chat_history. This unifies patterns and reduces mental overhead.

Details

• Action required: migrate to the new method names
• Clearer contracts for history retrieval and observability

Who this is for: Teams managing conversation history, logging, or analytics.