Fix getBytesRef in ConstantBytesRefVector (elastic#136445)

We should not return the BytesRef directly from ConstantBytesRefBlock, 
but instead copy its slice to a scratch buffer. Although current usage
does not modify the offset and length, the contract should allow callers
to change these fields, as long as the content of the bytes array
remains unchanged. The usage pattern below should be fine, but currently
it can change ConstantBytesRefBlock.

Author: dnhatn

x-pack/plugin/esql/compute/src/main/generated-src/org/elasticsearch/compute/data/ConstantBytesRefVector.java

@@ -40,11 +40,15 @@ $endif$
 
     @Override
 $if(BytesRef)$
-    public BytesRef getBytesRef(int position, BytesRef ignore) {
+    public BytesRef getBytesRef(int position, BytesRef scratch) {
+        scratch.bytes = value.bytes;
+        scratch.offset = value.offset;
+        scratch.length = value.length;
+        return scratch;
 $else$
     public $type$ get$Type$(int position) {
-$endif$
         return value;
+$endif$
     }
 
     @Override

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/X-ConstantVector.java.st

@@ -818,6 +818,12 @@ public void testConstantBytesRefBlock() {
             assertInsertNulls(block);
             assertDeepCopy(block);
             releaseAndAssertBreaker(block);
+            // modify the offset
+            var v0 = block.getBytesRef(randomInt(positionCount), new BytesRef());
+            var v1 = block.getBytesRef(randomInt(positionCount), new BytesRef());
+            v1.length = 0;
+            var v2 = block.getBytesRef(randomInt(positionCount), new BytesRef());
+            assertThat(v2, equalTo(v0));
         }
     }