fix(go): cache mileage after successful write

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

@@ -3,6 +3,7 @@ package stats
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"strings"
 	"testing"
 	"time"
@@ -174,6 +175,30 @@ func TestWriterSkipsConsecutiveDuplicateMileageSamples(t *testing.T) {
 	}
 }
 
+func TestWriterDoesNotCacheMileageWhenUpsertFails(t *testing.T) {
+	exec := &recordingExec{errs: []error{errors.New("mysql unavailable"), nil}}
+	writer := NewWriter(exec, time.FixedZone("Asia/Shanghai", 8*3600))
+	event := envelope.FrameEnvelope{
+		Protocol:    envelope.ProtocolJT808,
+		VIN:         "LNBVIN00000000001",
+		EventTimeMS: time.Date(2026, 7, 1, 9, 0, 0, 0, time.FixedZone("Asia/Shanghai", 8*3600)).UnixMilli(),
+		Fields: map[string]any{
+			envelope.FieldTotalMileageKM: 10241.2,
+		},
+	}
+
+	if err := writer.Append(context.Background(), event); err == nil {
+		t.Fatalf("first Append() error = nil, want mysql error")
+	}
+	if err := writer.Append(context.Background(), event); err != nil {
+		t.Fatalf("retry Append() error = %v", err)
+	}
+
+	if len(exec.calls) != 2 {
+		t.Fatalf("exec calls = %d, want 2", len(exec.calls))
+	}
+}
+
 func TestWriterKeepsOnlyLatestDateInMileageCache(t *testing.T) {
 	exec := &recordingExec{}
 	writer := NewWriter(exec, time.FixedZone("Asia/Shanghai", 8*3600))
@@ -209,9 +234,15 @@ type execCall struct {
 
 type recordingExec struct {
 	calls []execCall
+	errs  []error
 }
 
 func (e *recordingExec) ExecContext(_ context.Context, query string, args ...any) (sql.Result, error) {
 	e.calls = append(e.calls, execCall{query: query, args: args})
+	if len(e.errs) > 0 {
+		err := e.errs[0]
+		e.errs = e.errs[1:]
+		return nil, err
+	}
 	return nil, nil
 }