feat!: update ServerHandler and ServerHandlerCore traits

[hashemix]

📌 Summary

This PR refactors the signature of handler functions in ServerHandler and ServerHandlerCore to accept an Arc instead of a &dyn McpServer.
This PR also refactors the start function in server_runtime.rs to prevent the stream loop from being blocked by handle_message calls.

✨ Changes Made

• Updated handler functions to accept an Arc<dyn McpServer> instead of a &dyn McpServer
• Removed on_server_started in favor of on_initialized
• Unblock Stream Loop in start Function with Task Spawning

⚠ BREAKING CHANGE:

• Any implementations or overrides of the ServerHandler and ServerHandlerCore functions must update their signatures to accept Arc<dyn McpServer> instead of &dyn McpServer
• on_initialized() should be used instead of on_server_started()