Fix: Retrieve topic_prefix from config_entry for set_config_key service

[4homeassistant]

Fixes #159

Problem

The set_config_key service was using the hardcoded DEFAULT_TOPIC_PREFIX instead of reading the actual topic_prefix from the config entry. This caused the service to fail when the user's configured topic_prefix differed from the default value.

After version 0.24, the DEFAULT_TOPIC_PREFIX was changed from /go-eCharger to go-eCharger (without leading slash), breaking the service for users who had the leading slash in their configuration.

Solution

• Store config entries in hass.data for service access
• Retrieve topic_prefix from config_entry instead of using hardcoded default
• Add debug logging to show the actual topic being published to
• Add cleanup of stored config entries on unload

Testing

Tested successfully with topic_prefix configurations with leading slash: /go-eCharger

The service now correctly uses the user's configured topic_prefix regardless of the default value.

[4homeassistant]

Hey @syssi, I developed and tested a fix for that issue with the help of Perplexity and Claude. I am not really aware of the code but tried a vibe coding approach and tested it on my own installation. The concept of the change makes sense to me. What do you say?