fix(slack): ignore stale thread sessions when gating thread-context reseed

A session key that exists in the store but would be rolled to a fresh
session by the reset policy (daily/idle/suspended) is not an active
session. Treating it as active suppressed the first-turn Slack
thread-history reseed after reset (#55239).

_has_active_session_for_thread() now consults SessionStore._should_reset
so a stale entry gates like a missing one, letting _fetch_thread_context
reseed the fresh session with recent thread history.

Fixes #55239.

Salvaged from #55240 by @ooiuuii.

tests/gateway/test_slack_approval_buttons.py

@@ -933,6 +933,34 @@ class TestSessionKeyFix:
         )
         assert result is False
 
+    def test_stale_session_returns_false(self):
+        """A session key that exists but would be rolled by the reset policy
+        must NOT count as active — otherwise the reset-time first turn skips
+        the thread-history reseed (#55239)."""
+        adapter = _make_adapter()
+
+        class Store:
+            config = MagicMock()
+            config.group_sessions_per_user = False
+            config.thread_sessions_per_user = False
+            _entries = {
+                "agent:main:slack:group:C1:1000.0": MagicMock()
+            }
+
+            def _ensure_loaded(self):
+                return None
+
+            def _should_reset(self, entry, source):
+                return "idle"
+
+        adapter._session_store = Store()
+
+        result = adapter._has_active_session_for_thread(
+            channel_id="C1", thread_ts="1000.0", user_id="U123"
+        )
+
+        assert result is False
+
 
 # ===========================================================================
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