fix(protocol): harden gateway contract validation

tests/go/protocol_test.go

@@ -141,3 +141,37 @@ func TestGatewayClipboardAuditIsMetadataOnlyAndStrict(t *testing.T) {
 		}
 	}
 }
+
+func TestGatewayClipboardTextMeasuresDecodedUTF8Bytes(t *testing.T) {
+	for name, text := range map[string]string{
+		"ascii-boundary": strings.Repeat("a", 65536),
+		"utf8-boundary":  strings.Repeat("é", 32768),
+		"escape-heavy":   strings.Repeat(`"`, 32768),
+	} {
+		t.Run(name, func(t *testing.T) {
+			value := protocol.GatewayClipboardText{
+				Direction: "client_to_provider",
+				Text:      text,
+				Encoding:  "utf-8",
+				LoopToken: "abcdefghijklmnop",
+			}
+			encoded, err := protocol.EncodeGatewayClipboardText(value)
+			if err != nil {
+				t.Fatalf("EncodeGatewayClipboardText() error = %v", err)
+			}
+			decoded, err := protocol.DecodeGatewayClipboardText(encoded)
+			if err != nil || decoded.Text != text {
+				t.Fatalf("DecodeGatewayClipboardText() = %d bytes, %v", len(decoded.Text), err)
+			}
+		})
+	}
+	tooLarge := protocol.GatewayClipboardText{
+		Direction: "client_to_provider",
+		Text:      strings.Repeat("a", 65537),
+		Encoding:  "utf-8",
+		LoopToken: "abcdefghijklmnop",
+	}
+	if _, err := protocol.EncodeGatewayClipboardText(tooLarge); err == nil {
+		t.Fatal("EncodeGatewayClipboardText() accepted 65,537 decoded UTF-8 bytes")
+	}
+}