fix(encode): fall back from the literal multiline form when unsafe

Assisted-by: GLM 5.3

encode_test.go

@@ -267,6 +267,38 @@ func TestEncoderUseLiteralMultilineThresholdZero(t *testing.T) {
 	}
 }
 
+func TestEncoderLiteralMultilineFallsBackWhenUnsafe(t *testing.T) {
+	// The literal form carries the value verbatim, so content it cannot
+	// represent must fall back to the escaped basic string instead of
+	// producing output that does not re-parse.
+	cases := []struct {
+		name string
+		in   string
+	}{
+		{"embedded delimiter", "before ''' after\nsecond line"},
+		{"control character", "a\x01b\nsecond"},
+		{"delete character", "a\x7fb\nsecond"},
+		{"lone carriage return", "first\rsecond\nthird"},
+	}
+	for _, c := range cases {
+		out, err := NewEncoder().UseLiteralMultiline(5).Marshal(map[string]any{"s": c.in})
+		if err != nil {
+			t.Fatalf("%s: marshal: %v", c.name, err)
+		}
+		if !bytes.HasPrefix(out, []byte("s = \"")) {
+			t.Errorf("%s: expected the basic quoted form, got:\n%s", c.name, out)
+		}
+		re, err := Parse(out)
+		if err != nil {
+			t.Errorf("%s: re-parse: %v\ndoc:\n%s", c.name, err, out)
+			continue
+		}
+		if re["s"] != c.in {
+			t.Errorf("%s: round-trip changed the value: %q", c.name, re["s"])
+		}
+	}
+}
+
 // marshalerFunc adapts a plain function value to the Marshaler interface.
 // Tests use it to express "this field produces this TOML value" without a
 // dedicated struct definition.