Switch DTD retry delay to exponential backoff

Agent-Logs-Url: https://github.com/quickfix-j/quickfixj/sessions/bfbc9252-140b-44b9-a936-c3af5d32355e

Co-authored-by: chrjohn <6644028+chrjohn@users.noreply.github.com>

Author: Copilot

quickfixj-core/src/test/java/quickfix/MessageTest.java

@@ -1295,7 +1295,7 @@ private DataDictionary loadDataDictionaryWithExternalDtdRetry() throws ConfigErr
                     lastError = e;
                     if (attempt < EXTERNAL_DTD_LOAD_RETRIES) {
                         try {
-                            Thread.sleep((long) attempt * EXTERNAL_DTD_RETRY_DELAY_MILLIS);
+                            Thread.sleep((long) Math.pow(2, attempt - 1) * EXTERNAL_DTD_RETRY_DELAY_MILLIS);
                         } catch (InterruptedException interruptedException) {
                             Thread.currentThread().interrupt();
                             throw new ConfigError("Interrupted while retrying FIX_External_DTD.xml load", interruptedException);