Fix array index out of bounds in getAgentCaller method

The getAgentCaller method in Analyzer.java had a bug where it could
access frames.get(i + 1) when i = frames.size() - 1, causing an
IndexOutOfBoundsException.

Fixed by:
- Changing loop condition from i >= 0 to i >= 1
- Changing access from frames.get(i + 1) to frames.get(i - 1)

This ensures we never access beyond array bounds while maintaining
the correct logic to find the caller of agent methods.

Author: hoteye

benchmark-jfr-analyzer/src/main/java/io/opentelemetry/javaagent/benchmark/jfr/Analyzer.java

@@ -65,11 +65,11 @@ private static Set<String> getAgentCallers(List<RecordedEvent> events) {
   @Nullable
   private static String getAgentCaller(RecordedStackTrace stackTrace) {
     List<RecordedFrame> frames = stackTrace.getFrames();
-    for (int i = frames.size() - 1; i >= 0; i--) {
+    for (int i = frames.size() - 1; i >= 1; i--) {
       RecordedFrame frame = frames.get(i);
       RecordedMethod method = frame.getMethod();
       if (isAgentMethod(method)) {
-        RecordedFrame callerFrame = frames.get(i + 1);
+        RecordedFrame callerFrame = frames.get(i - 1);
         RecordedMethod callerMethod = callerFrame.getMethod();
         return getStackTraceElement(callerMethod, callerFrame);
       }