fix(review): unblock CI on the stale-unread-dot salvage (#4946)

Two CI-red issues, both fixed:

1. Lint (F401): the new tests/test_session_unread_dot_on_visit.py imported
   `re` but never used it. Dropped the import.

2. Test hang/timeout: loadSession() now routes its viewed-count/unread clear
   through the new _acknowledgeSessionVisit() helper, but the PRE-EXISTING
   tests/test_cross_session_message_load_isolation.py harness (which extracts
   loadSession and stubs its deps) didn't define it — so the extracted
   loadSession broke and the node driver hung to the 60s pytest-timeout.
   Stubbed _acknowledgeSessionVisit + _sessionVisitHasUnreadState to no-ops
   there, mirroring the pre-existing _setSessionViewedCount /
   _clearSessionCompletionUnread stubs they replaced. That harness covers
   cross-session load ordering + stale-reject, not unread-dot state, so a
   no-op stub preserves its intent; the real helper is exercised by the PR's
   own test_session_unread_dot_on_visit.py.

Verified: ruff forward gate clean; the timeout test passes in ~1s; the PR's
unread-dot suite 7/7.

tests/test_cross_session_message_load_isolation.py

@@ -225,6 +225,13 @@ function createEnvironment() {
   globalThis._clearSameSessionForceReloadHint = () => { clearHintCalls += 1; };
   globalThis._clearStuckSessionOnBoot = () => {};
   globalThis._setSessionViewedCount = () => {};
+  // #4946: loadSession() now routes its viewed-count/unread clear through
+  // _acknowledgeSessionVisit(). This harness exercises cross-session load
+  // ordering + stale-reject, not unread-dot state, so stub it (and its
+  // same-session-guard predicate) to no-ops — mirroring the pre-existing
+  // _setSessionViewedCount / _clearSessionCompletionUnread stubs it replaced.
+  globalThis._acknowledgeSessionVisit = () => {};
+  globalThis._sessionVisitHasUnreadState = () => false;
   globalThis.scheduleTodosRefresh = () => {};
   globalThis.startSessionStream = () => {};
   globalThis.syncTopbar = () => {};

tests/test_session_unread_dot_on_visit.py

@@ -28,7 +28,6 @@ Two invariants flagged in review are protected here and MUST NOT regress:
       unread authoritatively rather than doing ad-hoc DOM surgery (concern b).
 """
 import json
-import re
 import subprocess
 from pathlib import Path