S3: delay empty folder cleanup to prevent Spark write failures (#8970)

* S3: delay empty folder cleanup to prevent Spark write failures (#8963)

Empty folders were being cleaned up within seconds, causing Apache Spark
(s3a) writes to fail when temporary directories like _temporary/0/task_xxx/
were briefly empty.

- Increase default cleanup delay from 5s to 2 minutes
- Only process queue items that have individually aged past the delay
  (previously the entire queue was drained once any item triggered)
- Make the delay configurable via filer.toml:
  [filer.options]
  s3.empty_folder_cleanup_delay = "2m"

* test: increase cleanup wait timeout to match 2m delay

The empty folder cleanup delay was increased to 2 minutes, so the
Spark integration test needs to wait longer for temporary directories
to disappear.

* fix: eagerly clean parent directories after empty folder deletion

After deleting an empty folder, immediately try to clean its parent
rather than relying on cascading metadata events that each re-enter
the 2-minute delay queue. This prevents multi-minute waits when
cleaning nested temporary directory trees (e.g. Spark's _temporary
hierarchy with 3+ levels would take 6m+ vs near-instant).

Fixes the CI failure where lingering _temporary parent directories
were not cleaned within the test's 3-minute timeout.

test/s3/spark/issue_8285_repro_test.go

@@ -70,7 +70,8 @@ print("WRITE_COUNT=" + str(count))
 		"issue-8285/output/_temporary/0/",
 		"issue-8285/output/_temporary/0/_temporary/",
 	}
-	lingering := waitForObjectsToDisappear(t, env, "test", temporaryCandidates, 35*time.Second)
+	// Empty folder cleanup has a 2m default delay + 30s processor interval
+	lingering := waitForObjectsToDisappear(t, env, "test", temporaryCandidates, 3*time.Minute)
 	if len(lingering) > 0 {
 		t.Fatalf("issue #8285 regression detected: lingering temporary directories: %v", lingering)
 	}