Add Sombra Shades window shade sub-driver

[Woofser]

Summary

Adds a sub-driver to the zigbee-window-treatment driver for Sombra Shades
Zigbee window shade motors (manufacturer "Sombra Shades").

Details

• Position is reported and commanded through the standard Window Covering
  cluster (0% = open, 100% = closed); no inversion.
• Opening/Closing status is inferred from the direction of position change,
  so it works across Sombra motor variants (battery and DC) regardless of
  any manufacturer-specific motion cluster. A short settle timer emits the
  resting state once movement stops.
• Battery level via the Power Configuration cluster; power source via the
  Basic cluster.
• Registered through the existing lazy-load mechanism in sub_drivers.lua.

Testing

• Unit tests in test_zigbee_window_treatment_sombra.lua.
• Verified on physical Sombra hardware (battery and DC motors): open, close,
  pause, set level, preset, position reporting, battery, and power source.

[CLAassistant]

All committers have signed the CLA.

[nand-nor]

Is the change to the fingerprints file intentional? Im not sure this will be backwards compatible, depending on if there are devices in the field that already use the exiting fingerprint

[Woofser]

Good catch — that removal wasn't intentional. This branch predated the Sombra Shades/WM25/L-Z and Sombra Shades/SS25/L-Z fingerprints being added to main, so the diff was dropping them.

I've pushed a commit restoring both entries (unchanged, still mapped to window-treatment-battery), so the change is now purely additive — it only adds the Sombra Shades/SOMBRA/Z-M fingerprint (mapped to window-treatment-powerSource and handled by the new sombra sub-driver). No existing fingerprints are removed, so any field devices on WM25/L-Z or SS25/L-Z keep matching. Thanks for catching it!