Sends a message to an agent and returns the response. Supports both
blocking and streaming modes.

Blocking mode (stream: false, default): Returns the complete response
as a JSON object.

Streaming mode (stream: true): Returns a stream of Server-Sent Events.
Event types include:

• session_start — conversation session initialized
• agent_message — incremental text content
• tool_call — agent executing a tool/action
• tool_result — tool execution result
• knowledge_source — retrieved knowledge sources
• session_end — conversation complete with usage stats