Refactor WattPlan sources to use merged provider lists

[LordMike]

Summary

• refactor source acquisition to use a shared providers list and merged pre-normalization pipeline
• update config and options flows so source review persists providers and entity auto-detect resolves one provider per selected entity
• restore user-friendly auto-detect review text and keep legacy multi-entity runtime configs working by normalizing them into merged providers

Testing

• python3 -m py_compile src/custom_components/wattplan/config_flow.py src/custom_components/wattplan/source_pipeline.py src/custom_components/wattplan/source_provider.py tests/integration/test_source_flow_modifiers.py tests/integration/test_source_provider.py
• live Home Assistant verification in /mnt/n/Personal/hass-core for the source review flow and runtime regression check