Fix incorrect return type annotations in base_api

[johanzander]

Summary

• Fix plant_list return type from list[dict[str, Any]] to dict[str, Any] — it returns a dict with data and totalData keys
• Fix __get_all_devices return type from dict[str, Any] to list[dict[str, Any]] and default from {} to [] — it returns a list of devices

Context

Confirmed by existing examples:

• noah_example.py accesses plant_list["data"] and plant_list["totalData"] (dict, not list)
• tlx_example.py iterates for device in devices: directly over device_list() result (list, not dict)

Test plan

• Verify TLX/MIN examples still work
• Type checker passes

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