Fix Shelly Gen 2 power_get()

[vasileio]

Description
Fix KeyError in shelly_gen2 power_get() function due to incorrect JSON response parsing.  The current implementation attempts to access response["output"] directly, but the Shelly Gen 2 RPC API returns the output field nested inside a result object.

API Response Structure:

{
  "id": 1,
  "result": {
    "output": true,
    "apower": 0.0,
    "voltage": 245.3
  }
}

Fix:
Change r.json()["output"] to r.json()["result"]["output"] in line 30.

Testing:

• Verified with Shelly Plus Plug UK device
• All power operations (get/set/cycle) now work correctly
• Compatible with existing NetworkPowerPort configurations

Checklist

• Tests for the feature
• PR has been tested

[Emantor]

@aparcar please take a look.