code-to10 v6 A14 batch2: test E402/F401 cleanup plus touched-file residual E,F cleanup

(cherry picked from commit 1db02545f74c96b3b92be4fc32954659cec1f814)

Reconcile-conflict: A14b E402 imports referencing cloud.handler_core (does not exist here; core methods live in server.py) — kept HEAD imports with incoming's noqa: E402 markers, dropped the unused core_mod import (verified unused elsewhere in both files).

cloud/tests/test_coverage85_server.py

@@ -16,7 +16,6 @@ from decimal import Decimal
 from pathlib import Path
 
 try:
-    from cloud import server as _srv
     from cloud.server import (
         DbRateLimiter,
         RateLimiter,
@@ -77,7 +76,6 @@ try:
     )
 except ImportError:  # discover -s cloud/tests 时把项目根目录补进 sys.path
     sys.path.insert(0, str(Path(__file__).resolve().parents[2]))
-    from cloud import server as _srv
     from cloud.server import (
         DbRateLimiter,
         RateLimiter,
@@ -264,7 +262,6 @@ class PathHelpersTestCase(unittest.TestCase):
         self.assertIsNone(path)
         self.assertEqual(err[0], 409)
         # 异机 worktree 409。
-        import socket
         path, err = _task_worktree_for_files(
             _StubServer(tenants_root="/tmp/t"),
             {"id": "t1", "worktree_host": "definitely-not-this-host-xyz"})
@@ -421,7 +418,7 @@ class TraceReadTestCase(unittest.TestCase):
 
     def test_build_agent_chain(self):
         # 主任务 + 模型子代理 + 委派子任务（done）三路。
-        parent = self._write("task1.jsonl", [
+        self._write("task1.jsonl", [
             json.dumps({"id": "c1", "thread": "main-th", "kind": "message",
                         "text": "hi", "ts": 10}),
             json.dumps({"id": "c2", "thread": "sub-th", "kind": "message",