Fix plant_id type annotations in OpenApiV1 (int -> str)

[johanzander]

Follow-up to the py.typed typing added in 2.2.0. A few annotations didn't match the actual runtime shapes / call sites, forcing downstream consumers (Home Assistant) into # type: ignore and Any. Fixing them at the source.

Fixes #157.

Changes

1. OpenApiV1 plant_id: int → str (all five methods: plant_details, plant_energy_overview, plant_power_overview, plant_energy_history, device_list, plus docstrings). Plant IDs are opaque string identifiers — this matches GrowattApi's own signatures and how callers pass them. The int annotations made mypy reject the (correct) str arguments in consumers.

2. GrowattApi.plant_list fallback default [] → {}. The return type is already annotated dict[str, Any] on master, but the fallback still returned a list when the back key is absent. {} matches the declared type and the real shape ({"data": [...], "totalData": {...}}).

Notes

• OpenApiV1.device_list keeps its # type: ignore[override]: it still returns dict while the base GrowattApi.device_list returns list, so it remains an intentional LSP divergence (two different APIs). The int→str change only aligns the parameter.
• The plant_list return type and docstring were already corrected to dict on master, so they're not part of this diff — only the fallback default remained.

Verification

• mypy --ignore-missing-imports growattServer/ — clean.
• ruff check growattServer/ — clean.