add testcase

Signed-off-by: jsvisa <delweng@gmail.com>

log/format_test.go

@@ -1,6 +1,7 @@
 package log
 
 import (
+	"log/slog"
 	"math/rand"
 	"testing"
 )
@@ -22,3 +23,63 @@ func BenchmarkPrettyUint64Logfmt(b *testing.B) {
 		sink = appendUint64(buf, rand.Uint64(), false)
 	}
 }
+
+// testStringer implements both TerminalStringer and fmt.Stringer
+type testStringer struct{}
+
+func (t testStringer) String() string {
+	return "full string representation"
+}
+
+func (t testStringer) TerminalString() string {
+	return "terminal string representation"
+}
+
+func TestFormatSlogValue(t *testing.T) {
+	tests := []struct {
+		name     string
+		level    slog.Level
+		value    slog.Value
+		expected string
+	}{
+		{
+			name:     "DEBUG level uses String()",
+			level:    slog.LevelDebug,
+			value:    slog.AnyValue(testStringer{}),
+			expected: `"full string representation"`,
+		},
+		{
+			name:     "TRACE level uses String()",
+			level:    LevelTrace,
+			value:    slog.AnyValue(testStringer{}),
+			expected: `"full string representation"`,
+		},
+		{
+			name:     "INFO level uses TerminalString()",
+			level:    slog.LevelInfo,
+			value:    slog.AnyValue(testStringer{}),
+			expected: `"terminal string representation"`,
+		},
+		{
+			name:     "WARN level uses TerminalString()",
+			level:    slog.LevelWarn,
+			value:    slog.AnyValue(testStringer{}),
+			expected: `"terminal string representation"`,
+		},
+		{
+			name:     "ERROR level uses TerminalString()",
+			level:    slog.LevelError,
+			value:    slog.AnyValue(testStringer{}),
+			expected: `"terminal string representation"`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := FormatSlogValue(tt.level, tt.value, nil)
+			if string(result) != tt.expected {
+				t.Errorf("FormatSlogValue() = '%v', want '%v'", string(result), tt.expected)
+			}
+		})
+	}
+}