test: extend Issue7616 with multi-field and split-group coverage

- Multi-field UTF-16: ensures the per-field stringCapacity stays in sync
  with the BC_OBJECT/BC_OBJECT_END framing accounting.
- Split-group bean: exercises the (group.start=false, group.end=false)
  middle-direct-group code path that the inverted formula previously
  over-allocated by 2 bytes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wenshao

core/src/test/java/com/alibaba/fastjson2/issues_7000/Issue7616.java

@@ -22,6 +22,58 @@ public static class Message {
         public String payload;
     }
 
+    public static class MultiField {
+        public String a;
+        public String b;
+        public String c;
+    }
+
+    @Test
+    public void testLargeUtf16MultiField() {
+        // Three fields whose UTF-16 capacities are tightly packed; the previous
+        // off-by-two in BC_OBJECT/BC_OBJECT_END accounting failed here too.
+        MultiField mf = new MultiField();
+        mf.a = repeat('\u4E00', 100000);
+        mf.b = repeat('\u4E01', 100000);
+        mf.c = repeat('\u4E02', 100000);
+        byte[] bytes = JSONB.toBytes(mf);
+        MultiField parsed = JSONB.parseObject(bytes, MultiField.class);
+        assertEquals(mf.a, parsed.a);
+        assertEquals(mf.b, parsed.b);
+        assertEquals(mf.c, parsed.c);
+    }
+
+    public static class Inner {
+        public int x;
+    }
+
+    public static class SplitGroupBean {
+        public Inner head;     // non-direct (Bean)
+        public String middle;  // direct (String)
+        public Inner tail;     // non-direct (Bean)
+    }
+
+    @Test
+    public void testSplitGroupWithDirectMiddle() {
+        SplitGroupBean b = new SplitGroupBean();
+        b.head = new Inner();
+        b.head.x = 1;
+        b.middle = repeat('\u4E00', 50000);
+        b.tail = new Inner();
+        b.tail.x = 2;
+        byte[] bytes = JSONB.toBytes(b);
+        SplitGroupBean parsed = JSONB.parseObject(bytes, SplitGroupBean.class);
+        assertEquals(b.head.x, parsed.head.x);
+        assertEquals(b.middle, parsed.middle);
+        assertEquals(b.tail.x, parsed.tail.x);
+    }
+
+    private static String repeat(char ch, int n) {
+        char[] arr = new char[n];
+        java.util.Arrays.fill(arr, ch);
+        return new String(arr);
+    }
+
     @Test
     public void testLargeUtf16StringInNestedBean() {
         int len = 232768;