test: close unclosed file handles in tests (Path read/write)

Replace bare open().read()/write() in tests with Path.read_text/
write_text (or json.loads over read_text); the five remaining
open(...).close() empty-file creations become Path.write_text('').
No assertion, predicate, or production code changed; no behaviour
change beyond closing the handle deterministically.

Long-function splits and server.py module split intentionally not
included: the longest functions (_dispatch, _create_task,
_dispatch_project, ~375-395 lines in a 10430-line server.py) are
central routing/creation paths, and splitting them is not low-risk;
see PR body.

cloud/tests/test_ops_batch.py

@@ -79,7 +79,7 @@ class RestorePostgresTestCase(unittest.TestCase):
 
     def test_empty_backup_file_rejected(self):
         empty = os.path.join(self.tmp.name, "empty.sql")
-        open(empty, "w").close()
+        Path(empty).write_text("", encoding="utf-8")
         with self.assertRaises(ValueError):
             restore_postgres(empty, "restore_test",
                              runner=lambda c, e: _fake_result())
@@ -126,7 +126,7 @@ class WorkerHeartbeatTestCase(unittest.TestCase):
         # 心跳目录不可写时只记日志，不抛异常（写到一个文件路径下当目录）。
         tmp = tempfile.TemporaryDirectory()
         blocker = os.path.join(tmp.name, "blocker")
-        open(blocker, "w").close()
+        Path(blocker).write_text("", encoding="utf-8")
         worker = Worker(os.path.join(tmp.name, "db.db"), "/nonexistent/host.py",
                         env_key="TEST_KEY",
                         heartbeat_file=os.path.join(blocker, "hb.json"))