fixing SamplingServiceTests.testClusterChanged() (#136489)

This makes two fixes to SamplingServiceTests.testClusterChanged():

1. It prevents the test from running when the random sampling feature is not present
2. It avoids an edge case where the index request is larger than the maximum size of the entire sample

Closes #136450

Author: masseyke

server/src/test/java/org/elasticsearch/ingest/SamplingServiceTests.java

@@ -261,6 +261,7 @@ public void testMaybeSampleMaxSize() {
     }
 
     public void testClusterChanged() {
+        assumeTrue("Requires sampling feature flag", RANDOM_SAMPLING_FEATURE_FLAG);
         String indexName = randomIdentifier();
         SamplingService samplingService = getTestSamplingService();
         Map<String, Object> inputRawDocSource = randomMap(1, 100, () -> Tuple.tuple(randomAlphaOfLength(10), randomAlphaOfLength(10)));
@@ -276,7 +277,7 @@ public void testClusterChanged() {
                         new SamplingConfiguration(
                             1.0,
                             randomIntBetween(1, 1000),
-                            ByteSizeValue.ofBytes(randomLongBetween(100, 1000000)),
+                            ByteSizeValue.ofBytes(randomLongBetween(indexRequest.source().length(), 1_000_000)),
                             TimeValue.timeValueDays(randomIntBetween(1, 10)),
                             null
                         )
@@ -316,7 +317,7 @@ public void testClusterChanged() {
                         new SamplingConfiguration(
                             1.0,
                             1001,
-                            ByteSizeValue.ofBytes(randomLongBetween(100, 1000000)),
+                            ByteSizeValue.ofBytes(randomLongBetween(indexRequest.source().length(), 1_000_000)),
                             TimeValue.timeValueDays(randomIntBetween(1, 10)),
                             null
                         )