fix: cannot insert at UTF16String.length()

Author: itsaky

android-tree-sitter/src/main/java/com/itsaky/androidide/treesitter/string/UTF16String.java

@@ -105,6 +105,10 @@ public void append(String string, int fromIndex, int length) {
    * @param string The string to insert.
    */
   public void insert(int index, String string) {
+    if (index == length()) {
+      append(string);
+      return;
+    }
     checkIndex(index, length());
     Native.insert(this.pointer, string, index);
   }

android-tree-sitter/src/test/java/com/itsaky/androidide/treesitter/UTF16StringTest.java

@@ -174,4 +174,17 @@ public void testMultithreadedUse() throws InterruptedException {
       thread.join();
     }
   }
+
+  @Test
+  public void testInsertAtLength() {
+    final var str = UTF16StringFactory.newString("Hello");
+    assertThat(str).isNotNull();
+    assertThat(str.toString()).isEqualTo("Hello");
+    assertThat(str.length()).isEqualTo(5);
+
+    str.insert(5, " World!");
+    assertThat(str.length()).isEqualTo(12);
+    assertThat(str.toString()).isEqualTo("Hello World!");
+    str.close();
+  }
 }