Fix incorrect type annotations in OpenApiV1 plant_id and plant_list (#159)

* Align V1 plant_id annotations with the base class

GrowattApi annotates plant_id as str on every method that takes one, but
OpenApiV1 annotates it as int on plant_details, plant_energy_overview,
plant_power_overview, plant_energy_history and device_list.

All five only pass the value through to params={"plant_id": plant_id},
so str has always worked at runtime; only the annotation disagreed.
The mismatch forces callers that hold a plant id as a str to either
suppress arg-type errors or coerce with int(), which breaks on
non-numeric ids.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Return an empty dict from plant_list when "back" is absent

plant_list is annotated as returning dict[str, Any], but the fallback
when the response has no "back" key is [], so the empty case returns a
list and contradicts the annotation.

Callers that follow the annotation and do plant_info.get("data") hit an
AttributeError on that path, forcing an isinstance() guard purely to
defend against the default. plant_detail on the same endpoint family
already defaults to {}.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: johanzander