fix(stats): map protocol mileage fields

go/vehicle-gateway/internal/stats/daily_metric_test.go

@@ -40,6 +40,60 @@ func TestSamplesFromEnvelopeDerivesDailyMileageSample(t *testing.T) {
 	}
 }
 
+func TestSamplesFromEnvelopeMapsProtocolMileageFields(t *testing.T) {
+	loc := time.FixedZone("Asia/Shanghai", 8*3600)
+	tests := []struct {
+		name     string
+		protocol envelope.Protocol
+		fields   map[string]any
+		wantKM   float64
+	}{
+		{
+			name:     "gb32960",
+			protocol: envelope.ProtocolGB32960,
+			fields:   map[string]any{"gb32960.vehicle.total_mileage_km": "38587"},
+			wantKM:   38587,
+		},
+		{
+			name:     "jt808",
+			protocol: envelope.ProtocolJT808,
+			fields:   map[string]any{"jt808.location.total_mileage_km": "12432.4"},
+			wantKM:   12432.4,
+		},
+		{
+			name:     "yutong mqtt meters",
+			protocol: envelope.ProtocolYutongMQTT,
+			fields:   map[string]any{"yutong_mqtt.data.total_mileage": "120756000"},
+			wantKM:   120756,
+		},
+		{
+			name:     "yutong mqtt root meters",
+			protocol: envelope.ProtocolYutongMQTT,
+			fields:   map[string]any{"yutong_mqtt.root.data.total_mileage": float64(22858000)},
+			wantKM:   22858,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			samples, err := SamplesFromEnvelope(envelope.FrameEnvelope{
+				Protocol:    tt.protocol,
+				VIN:         "LNBVIN00000000001",
+				EventTimeMS: time.Date(2026, 7, 1, 8, 30, 0, 0, loc).UnixMilli(),
+				Fields:      tt.fields,
+			}, loc)
+			if err != nil {
+				t.Fatalf("SamplesFromEnvelope() error = %v", err)
+			}
+			if len(samples) != 1 {
+				t.Fatalf("sample count = %d", len(samples))
+			}
+			if samples[0].TotalMileageKM != tt.wantKM {
+				t.Fatalf("total mileage = %v, want %v", samples[0].TotalMileageKM, tt.wantKM)
+			}
+		})
+	}
+}
+
 func TestSamplesFromEnvelopeSkipsMissingVIN(t *testing.T) {
 	loc := time.FixedZone("Asia/Shanghai", 8*3600)
 	samples, err := SamplesFromEnvelope(envelope.FrameEnvelope{