feat: Add multi-inverter support with strict backwards compatibility

[ma27in]

This MR introduces the ability to configure multiple SAJ/Greenheiss inverters simultaneously within configuration.yaml, while safely preserving existing user configurations and historical data.

Key Changes:

• Multi-Inverter Support: Added a new optional provider_id to the PLATFORM_SCHEMA. When configured, this ID is injected into the sensor's name and unique_id (e.g., sensor.esolar_bir9_nowpower). This allows multiple integration blocks to run concurrently without entity ID collisions.
• Prefix Cleanup: Removed an accidental trailing space from the global SENSOR_PREFIX definition.
• Backwards Compatibility: To prevent orphaning existing users' sensors (which would wipe out their historical Energy Dashboard data), strict fallback logic was added to the SAJeSolarMeterSensor initialization. If provider_id is omitted (which defaults to None), the integration perfectly mirrors the legacy unique_id and naming structure, intentionally preserving the legacy spacing.

Testing:

• Verified that adding a provider_id correctly generates isolated sensors for multiple logins.
• Verified that omitting provider_id perfectly matches the legacy entity IDs, ensuring a seamless upgrade path for current users.