Switch empty-folder cleanup to bucket policy (#8292)

* Fix Spark _temporary cleanup and add issue #8285 regression test

* Generalize empty folder cleanup for Spark temp artifacts

* Revert synchronous folder pruning and add cleanup diagnostics

* Add actionable empty-folder cleanup diagnostics

* Fix Spark temp marker cleanup in async folder cleaner

* Fix Spark temp cleanup with implicit directory markers

* Keep explicit directory markers non-implicit

* logging

* more logs

* Switch empty-folder cleanup to bucket policy

* Seaweed-X-Amz-Allow-Empty-Folders

* less logs

* go vet

* less logs

* refactoring

weed/util/buckets.go

@@ -0,0 +1,33 @@
+package util
+
+import "strings"
+
+// ExtractBucketPath returns the bucket path under basePath that contains target.
+// If requireChild is true, the target must include additional segments beyond the bucket itself.
+func ExtractBucketPath(basePath, target string, requireChild bool) (string, bool) {
+	cleanBase := strings.TrimSuffix(basePath, "/")
+	if cleanBase == "" {
+		return "", false
+	}
+
+	prefix := cleanBase + "/"
+	if !strings.HasPrefix(target, prefix) {
+		return "", false
+	}
+
+	rest := strings.TrimPrefix(target, prefix)
+	if rest == "" {
+		return "", false
+	}
+
+	bucketName, _, found := strings.Cut(rest, "/")
+	if bucketName == "" {
+		return "", false
+	}
+
+	if requireChild && !found {
+		return "", false
+	}
+
+	return prefix + bucketName, true
+}