RepositoryAnalysisFailureIT: fix testFailsOnWriteException (#126750)

With the addition of copy coverage in the repository analyzer,
blob count is no longer 1:1 with blob analyzer request count: requests
that create a copy count as two blobs. This can cause
testFailsOnWriteException to sometimes fail, because this test randomly
injects a failure somewhere between the first and blobCounth request,
which may never happen if enough of the requests create copies.

This simple fix is to inject the failure within blobCount/2 requests,
which we will see even if every request generates a copy. An alternative
could be to add a knob to the request to disallow copies and use that
during this test.

Closes #126747

Author: bcully

muted-tests.yml

@@ -408,9 +408,6 @@ tests:
 - class: org.elasticsearch.xpack.ilm.TimeSeriesDataStreamsIT
   method: testShrinkActionInPolicyWithoutHotPhase
   issue: https://github.com/elastic/elasticsearch/issues/126746
-- class: org.elasticsearch.repositories.blobstore.testkit.analyze.RepositoryAnalysisFailureIT
-  method: testFailsOnWriteException
-  issue: https://github.com/elastic/elasticsearch/issues/126747
 - class: org.elasticsearch.xpack.test.rest.XPackRestIT
   method: test {p0=transform/transforms_start_stop/Test start/stop/start continuous transform}
   issue: https://github.com/elastic/elasticsearch/issues/126755

x-pack/plugin/snapshot-repo-test-kit/src/internalClusterTest/java/org/elasticsearch/repositories/blobstore/testkit/analyze/RepositoryAnalysisFailureIT.java

@@ -234,7 +234,9 @@ public void testFailsOnWriteException() {
         request.maxBlobSize(ByteSizeValue.ofBytes(10L));
         request.abortWritePermitted(false);
 
-        final CountDown countDown = new CountDown(between(1, request.getBlobCount()));
+        // requests that create copies count as two blobs. Halving the count ensures that we trigger the disruption
+        // even if every request is a copy
+        final CountDown countDown = new CountDown(between(1, request.getBlobCount() / 2));
 
         blobStore.setDisruption(new Disruption() {