fix(build): register a2a_response in TOP_LEVEL_MODULES

The drift gate caught the new SSOT parser module — without registration
the wheel ships it un-rewritten and runtime imports fail. Same pattern
as inbox_uploads, a2a_tools_delegation, a2a_tools_rbac registrations.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/build_runtime_package.py

@@ -54,6 +54,7 @@ TOP_LEVEL_MODULES = {
     "a2a_client",
     "a2a_executor",
     "a2a_mcp_server",
+    "a2a_response",
     "a2a_tools",
     "a2a_tools_delegation",
     "a2a_tools_inbox",