[SPARK-54625][SQL] UTF8String#reverse should check offset and length on copying

### What changes were proposed in this pull request?
This PR aims to check offset and length on copying in `UTF8String#reverse`.
For details, see https://lists.apache.org/thread/d9pvkh3jbsq8lc33v75kmwq5wg57422h (Only PMC members can read with login).
To avoid performance regression, this PR choose to check offset and length rather than validate the input UTF-8 string.

### Why are the changes needed?
For safety.

### Does this PR introduce _any_ user-facing change?
Yes, but doesn't break compatibility.

### How was this patch tested?
Example queries mentioned in [this thread](https://lists.apache.org/thread/d9pvkh3jbsq8lc33v75kmwq5wg57422h) works even though the results are broken.
All the operation defined in `UTF8String` are expected to work correctly with valid UTF-8 strings so the broken results with invalid UTF-8 strings should be reasonable.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes #53366 from sarutak/fix-utf8-reverse.

Authored-by: Kousuke Saruta <sarutak@amazon.co.jp>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: sarutak

common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -1160,9 +1160,10 @@ public UTF8String reverse() {
 
     int i = 0; // position in byte
     while (i < numBytes) {
-      int len = numBytesForFirstByte(getByte(i));
+      int len = Math.min(numBytesForFirstByte(getByte(i)), numBytes);
+      int targetOffset = Math.max(result.length - i - len, 0);
       copyMemory(this.base, this.offset + i, result,
-        BYTE_ARRAY_OFFSET + result.length - i - len, len);
+        BYTE_ARRAY_OFFSET + targetOffset, len);
 
       i += len;
     }