Use ConfigEntry.runtime_data for integration runtime state

[MTrab]

Summary

• migrate integration runtime storage from hass.data[DOMAIN][entry_id] to typed ConfigEntry.runtime_data
• add WebastoRuntimeData and WebastoConfigEntry typing in __init__.py
• update setup/unload/reload lifecycle to use entry.runtime_data
• update all platforms and diagnostics to read coordinator from entry.runtime_data.coordinator
• remove now-obsolete coordinator/devices/update-listener constants
• update affected unit tests for the new runtime-data model

Why

• aligns integration with current Home Assistant guidance for storing runtime data
• improves type safety and avoids ad-hoc global storage in hass.data

Test strategy

• ruff check custom_components/webastoconnect tests
• python3 -m pytest -q tests

Configuration changes

• none