Support new Copilot message payloads

[Copilot]

Copilot output parsing was dropping valid messages for session.mcp_servers_loaded, assistant.reasoning_delta, and assistant.message_start payloads. These schema mismatches caused noisy “Failed to parse copilot message” logs and lost event data.

• Message schema updates

  • Accept MCP server entries without source
  • Accept assistant message starts without phase
  • Add assistant.reasoning_delta parsing

• Parser coverage

  • Add targeted tests for the reported payload shapes

{
  type: 'assistant.reasoning_delta',
  data: {
    reasoningId,
    deltaContent,
  },
}