Enhance cloud-call serialization to prevent active-device races

[MTrab]

Summary

• introduce a coordinator-level async lock and async_execute_cloud_call helper to serialize cloud operations
• route coordinator polling (cloud.update) and write commands (switch/number) through the same serialized execution path
• add race-focused unit coverage for serialized cloud calls and update existing write-path/coordinator tests

Why

• improves robustness in concurrent poll + service-call scenarios where the backend active device context is mutable

Test strategy

• python3 -m pytest -q tests/test_api_coordinator.py tests/test_write_updates_without_refresh.py tests/test_coordinator_cloud_lock.py

Known limitations

• serialization is scoped per config entry/coordinator; multiple entries are still independent
• this change does not alter pywebasto internals, it enforces ordering at integration level

Configuration changes

• none