Refactor handle_delete to use extract_session_id

## Motivation and Context

`handle_post` and `handle_get` already use this helper, but
`handle_delete` inlined `request.env["HTTP_MCP_SESSION_ID"]`.
This aligns all three handlers consistently.

## How Has This Been Tested?

It has passed existing tests.

## Breaking Changes

None.

Author: koic

lib/mcp/server/transports/streamable_http_transport.rb

@@ -426,7 +426,7 @@ def handle_delete(request)
             return success_response
           end
 
-          return missing_session_id_response unless (session_id = request.env["HTTP_MCP_SESSION_ID"])
+          return missing_session_id_response unless (session_id = extract_session_id(request))
           return session_not_found_response unless session_exists?(session_id)
 
           protocol_version_error = validate_protocol_version_header(request)