Fix backing index pattern for system data streams to include a multiple migrated prefix (elastic#125598)

There is a theoretical possibility of having an index backing a system data stream with a name starting with .migrated-migrated-. This currently results in an error when accessing the data stream. This commit fixes the issue.

Author: alexey-ivanov-es

server/src/main/java/org/elasticsearch/indices/SystemDataStreamDescriptor.java

@@ -152,7 +152,7 @@ public String getBackingIndexPattern() {
     }
 
     private static String backingIndexPatternForDataStream(String dataStream) {
-        return ".(migrated-)?[fd]s-" + dataStream + "-*";
+        return ".(migrated-){0,}[fd]s-" + dataStream + "-*";
     }
 
     @Override

server/src/test/java/org/elasticsearch/indices/SystemIndicesTests.java

@@ -320,6 +320,10 @@ public void testSystemDataStreamPattern() {
             equalTo(true)
         );
         assertThat(systemIndices.isSystemIndexBackingDataStream(".migrated-ds-" + dataStreamName + "-2025.03.07-000001"), equalTo(true));
+        assertThat(
+            systemIndices.isSystemIndexBackingDataStream(".migrated-migrated-ds-" + dataStreamName + "-2025.03.07-000001"),
+            equalTo(true)
+        );
         assertThat(systemIndices.isSystemIndexBackingDataStream(".migrated-" + dataStreamName + "-2025.03.07-000001"), equalTo(false));
         assertThat(systemIndices.isSystemIndexBackingDataStream(dataStreamName), equalTo(false));
         assertThat(systemIndices.isSystemIndexBackingDataStream(dataStreamName + "-2025.03.07-000001"), equalTo(false));