br/pkg/stream: preserve m2's IngestedSstPaths in MergeMigrations

MergeMigrations only appended m1's IngestedSstPaths, silently dropping
m2's. During truncate (MergeAndMigrateTo -> processExtFullBackup), every
merged layer's ext_backups/ directory became invisible to the cleanup
logic and stayed on storage forever. Today this is a storage leak; once
lease-based lock expiration lands, an auto-reclaimed read lock followed
by a truncate can strand SSTs that PiTR still needs.

ref #67819

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: RidRisR

br/pkg/stream/stream_metas.go

@@ -614,6 +614,7 @@ func MergeMigrations(m1 *pb.Migration, m2 *pb.Migration) *pb.Migration {
 	out.DestructPrefix = append(out.DestructPrefix, m1.GetDestructPrefix()...)
 	out.DestructPrefix = append(out.DestructPrefix, m2.GetDestructPrefix()...)
 	out.IngestedSstPaths = append(out.IngestedSstPaths, m1.GetIngestedSstPaths()...)
+	out.IngestedSstPaths = append(out.IngestedSstPaths, m2.GetIngestedSstPaths()...)
 	return out
 }
 

br/pkg/stream/stream_metas_test.go

@@ -2992,3 +2992,15 @@ func TestGroupedExtFullBackup(t *testing.T) {
 		})
 	}
 }
+
+func TestMergeMigrationsPreservesIngestedSstPaths(t *testing.T) {
+	m1 := mig(mExtFullBackup("base/a", "base/b"))
+	m2 := mig(mExtFullBackup("layer/c", "layer/d"))
+
+	merged := MergeMigrations(m1, m2)
+
+	require.ElementsMatch(t,
+		[]string{"base/a", "base/b", "layer/c", "layer/d"},
+		merged.IngestedSstPaths,
+	)
+}