Hide runtime action envelope dispatch helper

[shuxueshuxue]

Summary

• make runtime action envelope dispatch an internal helper
• add a boundary guard so runtime_action.py does not publish that helper as public surface

Verification

• red first: boundary test failed while dispatch_runtime_action_envelope was exported
• uv run pytest tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py::test_runtime_action_adapters_do_not_export_envelope_dispatch_helpers tests/Unit/backend/web/services/test_runtime_action_dispatcher.py tests/Unit/integration_contracts/test_threads_router.py -q
• uv run ruff check backend/threads/chat_adapters/runtime_action.py tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py
• uv run pyright backend/threads/chat_adapters/runtime_action.py tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py
• uv run pytest tests/Unit/backend tests/Unit/integration_contracts -q