test(file-sync): patch module-level _monotonic alias instead of shared stdlib time module

Follow-up for salvaged PR #39946: file_sync.py already aliases time.sleep
as _sleep specifically to avoid tests mutating the shared stdlib module
object. Apply the same convention to the rate-limit clock (_monotonic)
and point the new regression test at it.

tests/tools/test_file_sync.py

@@ -236,7 +236,7 @@ class TestRateLimiting:
         from tools.environments import file_sync
 
         clock = {"t": 1000.0}
-        monkeypatch.setattr(file_sync.time, "monotonic", lambda: clock["t"])
+        monkeypatch.setattr(file_sync, "_monotonic", lambda: clock["t"])
 
         upload = MagicMock(side_effect=RuntimeError("transport down"))
         mgr = FileSyncManager(

tools/environments/file_sync.py

@@ -35,6 +35,9 @@ logger = logging.getLogger(__name__)
 # ``time.sleep`` globally because ``time`` is the module object; under xdist
 # that lets unrelated background threads inflate retry-test call counts.
 _sleep = time.sleep
+# Same rationale for the rate-limit clock: tests patch ``_monotonic``
+# instead of ``time.monotonic`` on the shared module object.
+_monotonic = time.monotonic
 
 _SYNC_INTERVAL_SECONDS = 5.0
 _FORCE_SYNC_ENV = "HERMES_FORCE_FILE_SYNC"
@@ -169,7 +172,7 @@ class FileSyncManager:
         On failure, state rolls back so the next cycle retries everything.
         """
         if not force and not os.environ.get(_FORCE_SYNC_ENV):
-            now = time.monotonic()
+            now = _monotonic()
             if now - self._last_sync_time < self._sync_interval:
                 return
 
@@ -193,7 +196,7 @@ class FileSyncManager:
         to_delete = [p for p in self._synced_files if p not in current_remote_paths]
 
         if not to_upload and not to_delete:
-            self._last_sync_time = time.monotonic()
+            self._last_sync_time = _monotonic()
             return
 
         # Snapshot for rollback (only when there's work to do)
@@ -227,7 +230,7 @@ class FileSyncManager:
                 self._pushed_hashes.pop(p, None)
 
             self._synced_files = new_files
-            self._last_sync_time = time.monotonic()
+            self._last_sync_time = _monotonic()
 
         except Exception as exc:
             self._synced_files = prev_files