feat(garmin): implement data-fetch methods via generic call dispatch

GetActivities/GetActivitySplits/GetActivityDetails/GetWorkoutByID now send
{"cmd":"call","params":{"method":...,"args":...}} instead of named MCP
tools. subprocessClient fully implements Client.

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

backend/internal/garmin/client_test.go

@@ -276,3 +276,121 @@ func TestSubprocessClient_CompleteMFA_Failed(t *testing.T) {
 		t.Errorf("CompleteMFA status = %v, want AuthFailed", res.Status)
 	}
 }
+
+func TestSubprocessClient_GetActivities_UsesGarminconnectKwargNames(t *testing.T) {
+	var gotMethod string
+	var gotArgs map[string]any
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		var p callParams
+		if err := json.Unmarshal(params, &p); err != nil {
+			t.Fatalf("unmarshal params: %v", err)
+		}
+		gotMethod, gotArgs = p.Method, p.Args
+		return fakeResult([]map[string]any{
+			{"activityId": float64(123), "activityType": map[string]any{"typeKey": "running"}},
+		})
+	})
+
+	activities, err := c.GetActivities(context.Background(), "2026-07-01", "2026-07-25", 0)
+	if err != nil {
+		t.Fatalf("GetActivities: %v", err)
+	}
+	if gotMethod != "get_activities_by_date" {
+		t.Errorf("method = %q, want get_activities_by_date", gotMethod)
+	}
+	if gotArgs["startdate"] != "2026-07-01" || gotArgs["enddate"] != "2026-07-25" {
+		t.Errorf("args = %+v, want startdate/enddate matching garminconnect's real kwargs", gotArgs)
+	}
+	if len(activities) != 1 || activities[0].ActivityID != 123 {
+		t.Fatalf("activities = %+v", activities)
+	}
+}
+
+func TestSubprocessClient_GetActivities_AppliesLimitClientSide(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		raw := make([]map[string]any, 5)
+		for i := range raw {
+			raw[i] = map[string]any{"activityId": float64(i)}
+		}
+		return fakeResult(raw)
+	})
+
+	activities, err := c.GetActivities(context.Background(), "2026-07-01", "2026-07-25", 2)
+	if err != nil {
+		t.Fatalf("GetActivities: %v", err)
+	}
+	if len(activities) != 2 {
+		t.Fatalf("len(activities) = %d, want 2", len(activities))
+	}
+}
+
+func TestSubprocessClient_GetActivitySplits_ParsesLapDTOsEnvelope(t *testing.T) {
+	var gotMethod string
+	var gotArgs map[string]any
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		var p callParams
+		json.Unmarshal(params, &p)
+		gotMethod, gotArgs = p.Method, p.Args
+		return fakeResult(map[string]any{
+			"activityId": float64(111),
+			"lapDTOs":    []map[string]any{{"lapIndex": float64(1), "distance": float64(1000)}},
+		})
+	})
+
+	splits, err := c.GetActivitySplits(context.Background(), 111)
+	if err != nil {
+		t.Fatalf("GetActivitySplits: %v", err)
+	}
+	if gotMethod != "get_activity_splits" || gotArgs["activity_id"] != "111" {
+		t.Errorf("method/args = %q/%+v, want get_activity_splits with activity_id=\"111\"", gotMethod, gotArgs)
+	}
+	if splits.ActivityID != 111 || len(splits.Laps) != 1 || splits.Laps[0].LapIndex != 1 {
+		t.Fatalf("splits = %+v", splits)
+	}
+}
+
+func TestSubprocessClient_GetActivityDetails_ParsesRawTelemetry(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		var p callParams
+		json.Unmarshal(params, &p)
+		if p.Method != "get_activity_details" || p.Args["activity_id"] != "222" {
+			t.Fatalf("unexpected call: %+v", p)
+		}
+		return fakeResult(map[string]any{
+			"activityId":            float64(222),
+			"metricDescriptors":     []map[string]any{{"key": "directHeartRate", "metricsIndex": float64(0)}},
+			"activityDetailMetrics": []map[string]any{{"metrics": []any{float64(101)}}},
+		})
+	})
+
+	details, err := c.GetActivityDetails(context.Background(), 222)
+	if err != nil {
+		t.Fatalf("GetActivityDetails: %v", err)
+	}
+	if details.ActivityID != 222 || len(details.MetricDescriptors) != 1 {
+		t.Fatalf("details = %+v", details)
+	}
+}
+
+func TestSubprocessClient_GetWorkoutByID_ParsesSegments(t *testing.T) {
+	c := newFakeWrapperClient(t, func(cmd string, params json.RawMessage) wireResponsePayload {
+		var p callParams
+		json.Unmarshal(params, &p)
+		if p.Method != "get_workout_by_id" || p.Args["workout_id"] != "333" {
+			t.Fatalf("unexpected call: %+v", p)
+		}
+		return fakeResult(map[string]any{
+			"workoutId":       float64(333),
+			"workoutName":     "Tempo",
+			"workoutSegments": []map[string]any{{"segmentOrder": float64(1), "workoutSteps": []any{}}},
+		})
+	})
+
+	workout, err := c.GetWorkoutByID(context.Background(), 333)
+	if err != nil {
+		t.Fatalf("GetWorkoutByID: %v", err)
+	}
+	if workout.WorkoutID != 333 || workout.WorkoutName != "Tempo" || len(workout.Segments) != 1 {
+		t.Fatalf("workout = %+v", workout)
+	}
+}