feat(garmin): mark a wrapper 404 with not_found in the error response

garminconnect's own GarminConnectNotFoundError already exists
specifically for this (its docstring: "so callers can now catch a
missing resource specifically, e.g. deleting an already-deleted
workout"), raised by connectapi() for any real HTTP 404. dispatch()
now surfaces that distinction as an extra not_found: true field
alongside the existing error string, so internal/garmin/client.go
(next commit) can tell a definitive 404 apart from a transient
failure.

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

backend/internal/garmin/pyscript/tests/test_wrapper.py

@@ -269,3 +269,25 @@ def test_authenticate_prevents_later_lazy_startup_login():
         wrapper.dispatch({"id": 34, "cmd": "call", "params": {"method": "x", "args": {}}})
 
     mock_startup.assert_not_called()
+
+
+def test_call_marks_not_found_error_specifically():
+    from garminconnect import GarminConnectNotFoundError
+
+    wrapper._auth_state = "authenticated"
+    wrapper._client = MagicMock()
+    wrapper._client.get_workout_by_id.side_effect = GarminConnectNotFoundError("API Error 404")
+    resp = wrapper.dispatch({
+        "id": 11, "cmd": "call", "params": {"method": "get_workout_by_id", "args": {"workout_id": "999"}},
+    })
+    assert resp == {"id": 11, "error": "API Error 404", "not_found": True}
+
+
+def test_call_does_not_mark_other_errors_as_not_found():
+    wrapper._auth_state = "authenticated"
+    wrapper._client = MagicMock()
+    wrapper._client.get_workout_by_id.side_effect = Exception("rate limited")
+    resp = wrapper.dispatch({
+        "id": 12, "cmd": "call", "params": {"method": "get_workout_by_id", "args": {"workout_id": "999"}},
+    })
+    assert resp == {"id": 12, "error": "rate limited"}