test✅: cover the operation log's truncation on a character boundary

SaveOperaLog shortens JsonResult to 100 characters before writing it.
#944 changed that from a byte cut to a rune cut, but no test referenced
it, so putting the byte cut back would turn nothing red.

The test drives the real SaveOperaLog with 150 three-byte characters and
reads the row back: the stored value must be valid UTF-8 and keep exactly
100 characters. With the byte cut restored it fails with an invalid
trailing byte and 34 characters kept.

app/admin/models/sys_opera_log_test.go

@@ -0,0 +1,53 @@
+package models
+
+import (
+	"strings"
+	"testing"
+	"unicode/utf8"
+
+	"github.com/glebarez/sqlite"
+	"github.com/go-admin-team/go-admin-core/v2/sdk"
+	"github.com/go-admin-team/go-admin-core/v2/storage/queue"
+	"gorm.io/gorm"
+)
+
+// SaveOperaLog shortens JsonResult before writing it. Cutting at a byte offset
+// splits a multi-byte character, and PostgreSQL and strict-mode MySQL reject the
+// resulting string, so the row is dropped and the only trace is a log line.
+func TestSaveOperaLogTruncatesOnCharacterBoundary(t *testing.T) {
+	const tenant = "opera-log-truncate-test"
+	db, err := gorm.Open(sqlite.Open("file:"+t.Name()+"?mode=memory&cache=shared"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("open db: %v", err)
+	}
+	if err := db.AutoMigrate(&SysOperaLog{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	sdk.Runtime.SetDbByTenant(tenant, db)
+	t.Cleanup(func() { sdk.Runtime.SetDbByTenant(tenant, nil) })
+
+	// 150 characters, 3 bytes each: byte 100 falls inside the 34th character.
+	long := strings.Repeat("中", 150)
+
+	msg := &queue.Message{}
+	msg.SetValues(map[string]interface{}{"title": "t", "jsonResult": long})
+	msg.SetPrefix(tenant)
+	if err := SaveOperaLog(msg); err != nil {
+		t.Fatalf("SaveOperaLog: %v", err)
+	}
+
+	var rows []SysOperaLog
+	if err := db.Find(&rows).Error; err != nil {
+		t.Fatalf("read back: %v", err)
+	}
+	if len(rows) != 1 {
+		t.Fatalf("want 1 row written, got %d", len(rows))
+	}
+	got := rows[0].JsonResult
+	if !utf8.ValidString(got) {
+		t.Errorf("stored JsonResult is not valid UTF-8: %q", got)
+	}
+	if n := utf8.RuneCountInString(got); n != 100 {
+		t.Errorf("want 100 characters kept, got %d", n)
+	}
+}