Trim notification batch envelope planner

[shuxueshuxue]

Summary

• remove the last batch envelope planner helper from runtime action adapters
• dispatch notification actions by planning one envelope at a time and lazily borrowing the runtime gateway only when an envelope exists
• add a boundary test preventing plan_runtime_*_envelopes helpers from becoming public adapter surface again

Verification

• RED: uv run pytest tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py -q
• uv run pytest tests/Unit/backend/web/services/test_workflow_event_notification.py::test_workflow_event_notification_fn_skips_gateway_when_no_runtime_recipients tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py tests/Unit/backend/web/services/test_runtime_notification_action.py -q
• uv run ruff check backend/threads/chat_adapters/runtime_notification_action.py tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py tests/Unit/backend/web/services/test_runtime_notification_action.py tests/Unit/backend/web/services/test_workflow_event_notification.py
• uv run ruff format --check backend/threads/chat_adapters/runtime_notification_action.py tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py
• uv run pyright backend/threads/chat_adapters/runtime_notification_action.py tests/Unit/backend/web/services/test_agent_runtime_gateway_protocol_boundary.py tests/Unit/backend/web/services/test_runtime_notification_action.py tests/Unit/backend/web/services/test_workflow_event_notification.py
• uv run pytest tests/Unit -q