Fix Solis case-insensitive SN matching in event handlers (#3433)

[springfall2008]

Summary

Fixes #3433 — solis.py event handlers still ignoring writes for inverters with uppercase hex digits in their serial number.

Root cause

HA normalises entity IDs to lowercase, so when number_event, select_event, or switch_event extract the serial number from the entity ID it arrives in lowercase (e.g. 111f33333333333). The previous fix (PR #3457) lowercased the SN when building entity IDs, but the event handlers still validated with a plain sn in self.inverter_sn check against self.inverter_sn which retains the original API casing (e.g. 111F33333333333). The comparison therefore failed for every write, producing the repeated warning:

Warn: Solis API: Unknown inverter 111f33333333333 in number_event

Fix

Add a find_inverter_by_sn() helper that performs a case-insensitive lookup and returns the canonical (API-casing) SN:

def find_inverter_by_sn(self, sn_from_entity_id):
    return next((sn for sn in self.inverter_sn if sn.lower() == sn_from_entity_id.lower()), None)

All three event handlers now call this helper. The returned SN is then used for Solis API payloads and internal dict keys (cached_values, charge_discharge_time_windows, etc.) so original casing is preserved where the API requires it.

Testing

Unit tests pass (./run_all --quick).