refactor(hsem): improve entity state fetching with unified error handling

[woopstar]

This refactor simplifies and stabilizes the _async_fetch_entity_states function by introducing a unified helper method for reading entity states.

Key improvements:

• Added _read_entity helper to standardize state fetching and error handling.
• Each entity is processed independently, ensuring one failure does not block the others.
• self._missing_input_entities_list is now reset at the start and filled with descriptive error messages.
• Clear distinction between missing entities and failed reads/conversions.
• Improved log messages to aid debugging and monitoring.
• Special handling for TOU charging/discharging periods remains, but now includes better error reporting.

Benefits:

• Cleaner, more maintainable code.
• Easier troubleshooting when entities are misconfigured or unavailable.
• Improved resilience: partial data is still processed even when some entities fail.

Fixes #153

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud