perf(sessions): skip counting entirely when transcript guards are disabled

With sessions.max_*_messages: 0 the guards previously still ran an
unbounded COUNT (full lineage for resume) — the exact pathological work
disabling them is meant to avoid. Live callers use the raise side
effect only, so return 0 without touching the messages table.

hermes_state.py

@@ -7929,9 +7929,11 @@ class SessionDB(SessionSearchMixin, SessionSchemaMixin, SessionPortabilityMixin)
         if max_messages < 0:
             raise ValueError("max_messages must be non-negative")
         if max_messages == 0:
-            # Guard disabled by config — never materialize an unbounded
-            # COUNT here; callers only need "safe", not an exact figure.
-            return self.get_resume_message_count(session_id)
+            # Guard disabled by config — skip counting entirely. Every live
+            # caller invokes this for its raise side effect and ignores the
+            # return value, and an unbounded lineage COUNT here would do the
+            # exact pathological work the disable exists to avoid.
+            return 0
         session_ids = self._session_lineage_root_to_tip(session_id)
         placeholders = ",".join("?" for _ in session_ids)
         with self._read_ctx() as conn:
@@ -7966,14 +7968,12 @@ class SessionDB(SessionSearchMixin, SessionSchemaMixin, SessionPortabilityMixin)
             max_messages = resolved_max_export_messages()
         if max_messages < 0:
             raise ValueError("max_messages must be non-negative")
+        if max_messages == 0:
+            # Guard disabled by config — skip the COUNT; live callers use
+            # this for its raise side effect only (and skip calling it
+            # entirely when the limit is 0).
+            return 0
         with self._read_ctx() as conn:
-            if max_messages == 0:
-                row = conn.execute(
-                    "SELECT COUNT(*) FROM messages "
-                    "WHERE session_id = ? AND active = 1",
-                    (session_id,),
-                ).fetchone()
-                return int(row[0] if row else 0)
             row = conn.execute(
                 "SELECT COUNT(*) FROM ("
                 "SELECT 1 FROM messages WHERE session_id = ? AND active = 1 LIMIT ?"

tests/test_hermes_state.py

@@ -3847,15 +3847,16 @@ class TestGetMessagesPagination:
         with pytest.raises(hermes_state.SessionExportTooLargeError):
             db.assert_export_safe("big", max_messages=2)
 
-        # ...but a config-resolved limit of 0 disables both guards and
-        # returns the true count without raising.
+        # ...but a config-resolved limit of 0 disables both guards: no raise,
+        # and no counting work at all (returns 0 — callers use the raise side
+        # effect only).
         monkeypatch.setattr(hermes_state, "resolved_max_resume_messages", lambda: 0)
         monkeypatch.setattr(hermes_state, "resolved_max_export_messages", lambda: 0)
-        assert db.assert_resume_safe("big") == 5
-        assert db.assert_export_safe("big") == 5
+        assert db.assert_resume_safe("big") == 0
+        assert db.assert_export_safe("big") == 0
         # An explicit 0 disables too, independent of config.
-        assert db.assert_resume_safe("big", max_messages=0) == 5
-        assert db.assert_export_safe("big", max_messages=0) == 5
+        assert db.assert_resume_safe("big", max_messages=0) == 0
+        assert db.assert_export_safe("big", max_messages=0) == 0
 
     def test_guard_limits_resolve_from_config_at_call_time(self, db, monkeypatch):
         db.create_session(session_id="cfg", source="cli")