Refactor handle_delete to use extract_session_id and missing_session_id_response

`handle_post` and `handle_get` already use these helper methods,
but `handle_delete` inlined the same logic.
This aligns all three handlers to use the shared helpers consistently.

Author: koic

lib/mcp/server/transports/streamable_http_transport.rb

@@ -154,13 +154,9 @@ def handle_delete(request)
             return success_response
           end
 
-          session_id = request.env["HTTP_MCP_SESSION_ID"]
+          session_id = extract_session_id(request)
 
-          return [
-            400,
-            { "Content-Type" => "application/json" },
-            [{ error: "Missing session ID" }.to_json],
-          ] unless session_id
+          return missing_session_id_response unless session_id
 
           cleanup_session(session_id)
           success_response