filer: keep the internal .system folder out of the per-bucket store path (#10248)

* filer: keep .system internal folder in the default SQL table

Bucket-table SQL stores read the first path segment under /buckets as a
bucket name. The ListBuckets owner index lives at /buckets/.system/..., so
every write there hit isValidBucket(".system") == false and returned
"invalid bucket name .system", flooding the filer log on the postgres/mysql
backends. Route dot-prefixed internal folders to the default table by their
full path, like any other non-bucket entry.

* filer: keep .system internal folder in the default leveldb3 DB

Same guard as the SQL stores: leveldb3 would otherwise open a separate DB
for the .system owner-index folder instead of keeping it in the default DB.

* filer: keep .system internal folder under the default ydb prefix

Skips a DescribeTable round trip per operation on the .system owner-index
path, which never resolves to a real bucket table.

* filer: keep .system internal folder in the default arangodb collection

Avoids creating a stray collection for the .system owner-index folder.

weed/filer/abstract_sql/abstract_sql_store.go

@@ -129,6 +129,14 @@ func (store *AbstractSqlStore) getTxOrDB(ctx context.Context, fullpath util.Full
 		shortPath = util.FullPath(bucketAndObjectKey[t:])
 	}
 
+	// Dot-prefixed entries directly under /buckets (e.g. .system) are internal
+	// folders, not S3 buckets; keep them in the default table by full path.
+	if strings.HasPrefix(bucket, ".") {
+		bucket = DEFAULT_TABLE
+		shortPath = fullpath
+		return
+	}
+
 	if isValidBucket(bucket) {
 		store.dbsLock.Lock()
 		defer store.dbsLock.Unlock()