feat: add PVForecastHomeAssistant provider (#1258)

* feat: add PVForecastHomeAssistant provider

Reads a PV forecast time series directly from a Home Assistant entity
attribute (matching the {"forecast": [{"datetime", "watts"}]} shape
already exposed by common HA PV forecast integrations, e.g. Helios
Forecast and Solcast) and feeds it into pvforecast_ac_power, following
the same self-polling provider pattern as PVForecastVrm.

Closes #1232.

* fix: use MagicMock instead of monkeypatched Response for mypy

requests.Response().json is a typed bound method; reassigning it to a
lambda fails mypy's method-assign check. Use MagicMock(spec=...) instead,
which mocks the response without fighting its static type.

* fix: address PR review on PVForecastHomeAssistant provider

Fixes two issues raised in review on PR #1258:
- _update_data() left stale pvforecast_ac_power values in place when a
  refreshed forecast was empty or shorter than a previous one; it now
  clears the active forecast window before writing and raises instead
  of silently no-op'ing when the response has no usable data.
- pvforecast.homeassistant.entity_id used a "select" widget with no
  manual-entry fallback in EOSdash, leaving it unusable in standalone
  mode where the entity list can't be resolved without SUPERVISOR_TOKEN;
  switched to a plain text field.

Also fills in the config docs and openapi.json for the new
pvforecast.homeassistant.* fields, which were missing from the
original commit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix: preserve retained forecast history when clearing stale entries

The previous fix cleared pvforecast_ac_power from start-of-day, but
PredictionProvider deliberately retains historical records back to
keep_datetime (prediction.historic_hours). Since Home Assistant
forecasts are future-only, that clear wiped out retained history
between midnight and the EMS start on every refresh.

Narrow the clear to [ems_start_datetime, end_datetime) - the actual
active forecast window, DST-adjusted - instead of the day boundary.

Addresses review feedback from @NormannK on PR #1258.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: Mathias <mathias@Mathiass-MacBook-Air.local>
Co-authored-by: Mathias <mathias@Mathiass-Air.localdomain>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

docs/_generated/configexample.md

@@ -244,6 +244,15 @@
                "token": "your-token",
                "site_id": 12345
            },
+           "homeassistant": {
+               "entity_id": "sensor.pv_forecast",
+               "attribute": "forecast",
+               "datetime_key": "datetime",
+               "value_key": "watts",
+               "value_unit": "W",
+               "base_url": null,
+               "token": null
+           },
            "pvlib": {},
            "pvnode": {
                "api_key": "",