Use missing_session_id_response method in handle_delete

## Motivation and Context

`handle_delete` had an inline error response for "Missing session ID" that
was identical to the existing `missing_session_id_response` helper method.
`handle_get` already uses this helper for the same purpose.

## How Has This Been Tested?

All existing tests pass. The behavior is unchanged.

## Breaking Change

None.

Author: koic

lib/mcp/server/transports/streamable_http_transport.rb

@@ -156,11 +156,7 @@ def handle_delete(request)
 
           session_id = request.env["HTTP_MCP_SESSION_ID"]
 
-          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