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_docker_sandbox.py

@@ -347,7 +347,7 @@ class TestDockerExtraMounts(DockerCommandTestCase):
             work = os.path.join(td, "work")
             os.makedirs(work)
             state = os.path.join(td, "host-state.json")
-            open(state, "w").write("{}")
+            Path(state).write_text("{}", encoding="utf-8")
             with mock.patch("cloud.sandbox.shutil.which",
                             return_value="/usr/bin/docker"):
                 ex = DockerExecutor("/fake/host.py", runner=_ok_runner_factory([]),