chore: guard against invalid UUIDs

Some integration tests failed w/ a BufferUnderflow exception
when run against v1.32.16, which we can handle more gracefully
by returning null from decodeUuid().

An invalid UUID is a server-side issue and we should not try to
decode it in the client.

Author: bevzzz

src/main/java/io/weaviate/client6/v1/api/collections/data/DeleteManyRequest.java

@@ -46,7 +46,7 @@ public static Rpc<DeleteManyRequest, WeaviateProtoBatchDelete.BatchDeleteRequest
           var objects = reply.getObjectsList()
               .stream()
               .map(obj -> new DeleteManyResponse.DeletedObject(
-                  ByteStringUtil.decodeUuid(obj.getUuid()).toString(),
+                  ByteStringUtil.decodeUuid(obj.getUuid()),
                   obj.getSuccessful(),
                   obj.getError()))
               .toList();

src/main/java/io/weaviate/client6/v1/internal/grpc/ByteStringUtil.java

@@ -16,11 +16,14 @@ private ByteStringUtil() {
   private static final ByteOrder BYTE_ORDER = ByteOrder.LITTLE_ENDIAN;
 
   /** Decode ByteString to UUID. */
-  public static UUID decodeUuid(ByteString bs) {
+  public static String decodeUuid(ByteString bs) {
+    if (bs.size() != Long.BYTES * 2) {
+      return null;
+    }
     var buf = ByteBuffer.wrap(bs.toByteArray());
     var most = buf.getLong();
     var least = buf.getLong();
-    return new UUID(most, least);
+    return new UUID(most, least).toString();
   }
 
   /** Encode float[] to ByteString. */

src/test/java/io/weaviate/client6/v1/internal/grpc/ByteStringUtilTest.java

@@ -49,10 +49,17 @@ public void test_decodeVector_2d() {
   public void test_decodeUuid() {
     byte[] bytes = { 38, 19, -74, 24, -114, -19, 73, 43, -112, -60, 47, 96, 83, -89, -35, -23 };
     String want = "2613b618-8eed-492b-90c4-2f6053a7dde9";
-    String got = ByteStringUtil.decodeUuid(ByteString.copyFrom(bytes)).toString();
+    String got = ByteStringUtil.decodeUuid(ByteString.copyFrom(bytes));
     Assertions.assertThat(got).isEqualTo(want);
   }
 
+  @Test
+  public void test_decodeUuid_bufferUnderflow() {
+    byte[] bytes = { 38, 19 }; // A valid UUID is exactly 16 bytes
+    String got = ByteStringUtil.decodeUuid(ByteString.copyFrom(bytes));
+    Assertions.assertThat(got).isNull();
+  }
+
   @Test
   public void test_decodeVector_1d_empty() {
     byte[] bytes = new byte[0];