fix: parse last_activity_date as datetime for HA timestamp sensor

SensorDeviceClass.TIMESTAMP requires a datetime object, not a string.
The value_fn was returning the raw ISO string from start_utc, causing
a ValueError on entity registration.

- Add _last_activity_datetime() helper using dt_util.parse_datetime
- Returns None for missing or unparseable dates (graceful degradation)
- Add tests for None start_utc and unparseable date strings

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mikejhill

custom_components/rouvy/sensor.py

@@ -5,6 +5,7 @@
 import logging
 from collections.abc import Callable
 from dataclasses import dataclass
+from datetime import datetime
 from typing import TYPE_CHECKING, Any
 
 from homeassistant.components.sensor import (
@@ -21,6 +22,7 @@
 )
 from homeassistant.core import HomeAssistant
 from homeassistant.helpers.entity_platform import AddEntitiesCallback
+from homeassistant.util import dt as dt_util
 
 from .api_client.models import Activity, ActivityTypeStats, RouvyCoordinatorData
 from .data import RouvyConfigEntry
@@ -55,6 +57,14 @@ def _last_activity(data: RouvyCoordinatorData) -> Activity | None:
     return None
 
 
+def _last_activity_datetime(data: RouvyCoordinatorData) -> datetime | None:
+    """Get the start time of the most recent activity as a datetime."""
+    act = _last_activity(data)
+    if act and act.start_utc:
+        return dt_util.parse_datetime(act.start_utc)
+    return None
+
+
 @dataclass(frozen=True, kw_only=True)
 class RouvySensorDescription(SensorEntityDescription):
     """Describe a Rouvy sensor."""
@@ -245,7 +255,7 @@ class RouvySensorDescription(SensorEntityDescription):
         key="last_activity_date",
         translation_key="last_activity_date",
         device_class=SensorDeviceClass.TIMESTAMP,
-        value_fn=lambda d: a.start_utc if (a := _last_activity(d)) else None,
+        value_fn=_last_activity_datetime,
     ),
     RouvySensorDescription(
         key="total_activities",

tests/test_ha_sensor.py

@@ -6,6 +6,8 @@
 
 from __future__ import annotations
 
+from datetime import UTC, datetime
+
 from custom_components.rouvy.api_client.models import (
     Activity,
     ActivitySummary,
@@ -649,7 +651,8 @@ def test_returns_start_utc(self) -> None:
         from custom_components.rouvy.sensor import SENSOR_DESCRIPTIONS
 
         desc = next(s for s in SENSOR_DESCRIPTIONS if s.key == "last_activity_date")
-        assert desc.value_fn(d) == "2026-04-01T07:30:00Z"
+        result = desc.value_fn(d)
+        assert result == datetime(2026, 4, 1, 7, 30, tzinfo=UTC)
 
     def test_returns_none_when_no_summary(self) -> None:
         d = _make_data_with_activities(summary=None)
@@ -658,6 +661,20 @@ def test_returns_none_when_no_summary(self) -> None:
         desc = next(s for s in SENSOR_DESCRIPTIONS if s.key == "last_activity_date")
         assert desc.value_fn(d) is None
 
+    def test_returns_none_when_start_utc_is_none(self) -> None:
+        d = _make_data_with_activities([_make_activity(start_utc=None)])
+        from custom_components.rouvy.sensor import SENSOR_DESCRIPTIONS
+
+        desc = next(s for s in SENSOR_DESCRIPTIONS if s.key == "last_activity_date")
+        assert desc.value_fn(d) is None
+
+    def test_returns_none_for_unparseable_date(self) -> None:
+        d = _make_data_with_activities([_make_activity(start_utc="not-a-date")])
+        from custom_components.rouvy.sensor import SENSOR_DESCRIPTIONS
+
+        desc = next(s for s in SENSOR_DESCRIPTIONS if s.key == "last_activity_date")
+        assert desc.value_fn(d) is None
+
 
 class TestTotalActivitiesSensor:
     """Verify total_activities sensor value extraction."""