fix(gateway): never prune sessions when active-process check fails

prune_old_entries' active-process guard failed open: when
has_active_processes_fn raised, the except block logged at debug and
fell through to the age check, so sessions with live background
processes attached could still be pruned — violating the documented
invariant that such sessions are never dropped. Add a continue so an
exception in the safety check fails safe (the entry is kept).

Commit 6b408e131 fixed the session_key/session_id mismatch in this
same guard but left the exception path failing open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gateway/session.py

@@ -2179,6 +2179,10 @@ class SessionStore:
                             "has_active_processes_fn raised during prune for %s: %s",
                             entry.session_key, exc,
                         )
+                        # Fail safe: if we can't tell whether a background
+                        # process is attached, keep the entry rather than
+                        # risk orphaning live work.
+                        continue
                 if entry.updated_at < cutoff:
                     removed_keys.append(key)
             for key in removed_keys:

tests/gateway/test_session_store_prune.py

@@ -165,6 +165,37 @@ class TestPruneBasics:
         assert removed == 1
         assert "active" not in store._entries
 
+    def test_prune_keeps_entry_when_active_check_raises(self, tmp_path):
+        """A failing active-process check must fail safe, not fail open.
+
+        If has_active_processes_fn raises, we can't tell whether a live
+        background process is attached — so the entry must be kept.
+        Previously the except block logged and fell through to the age
+        check, pruning the session anyway.
+        """
+        def _broken(session_key: str) -> bool:
+            raise RuntimeError("process registry unavailable")
+
+        store = _make_store(tmp_path, has_active_processes_fn=_broken)
+        store._entries["old"] = _entry("old", age_days=1000)
+
+        removed = store.prune_old_entries(max_age_days=90)
+
+        assert removed == 0
+        assert "old" in store._entries
+
+    def test_prune_removes_old_entry_when_active_check_returns_false(self, tmp_path):
+        """Sibling guard: a callback that cleanly reports no active process
+        must still allow the old entry to be pruned.
+        """
+        store = _make_store(tmp_path, has_active_processes_fn=lambda key: False)
+        store._entries["old"] = _entry("old", age_days=1000)
+
+        removed = store.prune_old_entries(max_age_days=90)
+
+        assert removed == 1
+        assert "old" not in store._entries
+
     def test_prune_does_not_write_disk_when_no_removals(self, tmp_path):
         """If nothing is evictable, _save() should NOT be called."""
         store = _make_store(tmp_path)