Allow ping requests before initialization

[eleftherias]

As per the MCP specification, ping requests should be allowed even before initialization:

The client SHOULD NOT send requests other than pings before the server has responded to the initialize request.
The server SHOULD NOT send requests other than pings and logging before receiving the initialized notification.

Currently, sending a ping request before initialization fails with Invalid request parameters.
The server logs show Failed to validate request: Received request before initialization was complete.