fix(jdbc): close the prepared-statement binding and metadata gaps

libsql-hrana-codec/src/main/java/com/github/magnusp/libsql/hrana/codec/DirectHranaJsonCodec.java

@@ -694,7 +694,16 @@ public void writeString(String s) {
                         } else if (c < 0x80) {
                             writeByte((byte) c);
                         } else {
-                            byte[] utf8 = String.valueOf(c).getBytes(StandardCharsets.UTF_8);
+                            String chunk;
+                            if (Character.isHighSurrogate(c)
+                                    && i + 1 < len
+                                    && Character.isLowSurrogate(s.charAt(i + 1))) {
+                                chunk = s.substring(i, i + 2);
+                                i++;
+                            } else {
+                                chunk = String.valueOf(c);
+                            }
+                            byte[] utf8 = chunk.getBytes(StandardCharsets.UTF_8);
                             ensureCapacity(count + utf8.length);
                             System.arraycopy(utf8, 0, buf, count, utf8.length);
                             count += utf8.length;

libsql-hrana-codec/src/test/java/com/github/magnusp/libsql/hrana/codec/DirectHranaJsonCodecTest.java

@@ -34,6 +34,20 @@ void testSerializePipelineRequestOneShot() throws IOException {
         assertThat(json).contains("\"value\":\"hello \\\"world\\\"\\nnewline\"");
     }
 
+    @Test
+    void testSerializeSupplementaryCharactersAsUtf8() throws IOException {
+        String supplementary = "\uD840\uDC40 and \uD880\uDC99";
+        Stmt stmt = new Stmt("SELECT '" + supplementary + "'", List.of(Value.of(supplementary)));
+        PipelineReqBody req = new PipelineReqBody(null, List.of(new StreamRequest.Execute(stmt)));
+
+        byte[] jsonBytes = codec.serializePipelineRequest(req);
+        String json = new String(jsonBytes, StandardCharsets.UTF_8);
+
+        assertThat(json).contains("\"sql\":\"SELECT '" + supplementary + "'\"");
+        assertThat(json).contains("\"value\":\"" + supplementary + "\"");
+        assertThat(json).doesNotContain("?");
+    }
+
     @Test
     void testDeserializePipelineResponse() throws IOException {
         String json = """