fix(core): Back up and restore FST buckets again

Since 27a1b2b ("chore(core): Flatten FST store code"), the bucket loop in
`FstStorePool::dump_action` checked the type and name of the collection
entry instead of the bucket entry. As a collection is a directory, every
bucket was skipped: `TRIGGER backup` answered `OK` but wrote no FST, and
`TRIGGER restore` restored none.

Add a test which backs up a consolidated FST and restores it.

core/tests/data_backup.rs

@@ -0,0 +1,46 @@
+// Sonic
+//
+// Fast, lightweight and schema-less search backend
+// Copyright: 2026, DualFroz <me@dualfroz.com>
+// License: Mozilla Public License v2.0 (MPL v2.0)
+
+//! Feature: Data backup
+
+mod common;
+
+use crate::common::util::unique_hex;
+use crate::common::*;
+
+/// Consolidated FST stores are backed up, and can be restored.
+#[test]
+fn test_fst_backup_and_restore() {
+    init_logging();
+    let executor = make_test_executor(|_| {});
+
+    exec!(executor -> PUSH "messages" "user:1" "chat:1" "Hello world");
+    exec!(executor -> TRIGGER consolidate);
+
+    let backup_path = std::path::Path::new(env!("CARGO_TARGET_TMPDIR"))
+        .join(unique_hex().unwrap())
+        .join("fst-backup");
+
+    executor.fst_pool.backup(&backup_path).unwrap();
+
+    let backup_files = std::fs::read_dir(&backup_path)
+        .unwrap()
+        .flat_map(|collection| std::fs::read_dir(collection.unwrap().path()).unwrap())
+        .map(|bucket| bucket.unwrap().path())
+        .collect::<Vec<_>>();
+    assert_eq!(backup_files.len(), 1, "got: {backup_files:?}");
+    assert!(
+        backup_files[0].to_string_lossy().ends_with(".fst.bck"),
+        "got: {backup_files:?}"
+    );
+
+    let restored_executor = make_test_executor(|_| {});
+
+    restored_executor.fst_pool.restore(&backup_path).unwrap();
+
+    let response = exec!(restored_executor -> LIST "messages" "user:1");
+    assert_contains!(response, ["hello", "world"]);
+}