fix(gateway): guard display config reads against present-but-null values

A profile config with a bare `display:` key (present-but-null) made
`user_config.get("display", {})` return None — the {} default only
applies when the key is missing — so the chained
`.get("memory_notifications")` in _wire_turn_agent_callbacks raised
AttributeError on every real gateway turn (Discord / cron). Oneshot
turns bypass this wiring, which masked the crash during smoke tests.

Use the same `or {}` guard the other gateway display readers
(display_config.py, runtime_footer.py) already apply, and fall back to
the documented default "on".

Fixes #105674

gateway/run_turn_runner.py

@@ -1176,7 +1176,9 @@ class TurnRunner:
                 if pdc is not None:
                     pdc[ctx.session_key] = bg_release
         # display.memory_notifications: off | on (generic "💾 Memory updated", default) | verbose.
-        mem_notif = ctx.user_config.get("display", {}).get("memory_notifications")
+        # `display:` present-but-null yields None, not the {} default (same `or {}` guard as
+        # display_config.py / runtime_footer.py).
+        mem_notif = (ctx.user_config.get("display") or {}).get("memory_notifications")
         if isinstance(mem_notif, bool):
             mem_notif = "on" if mem_notif else "off"
         agent.memory_notifications = str(mem_notif).lower() if mem_notif else "on"

tests/gateway/test_display_null_turn_wiring.py

@@ -0,0 +1,68 @@
+"""A profile config with `display: null` must not crash turn wiring.
+
+`user_config.get("display", {})` returns None when the key is present but null
+(bare `display:` in YAML), so the chained `.get("memory_notifications")` raised
+AttributeError on every real gateway turn (#105674). Oneshot turns bypass
+`_wire_turn_agent_callbacks`, which masked the regression during smoke tests.
+"""
+
+from __future__ import annotations
+
+import types
+
+from gateway.run_turn_runner import TurnRunner
+
+
+def _wire(user_config):
+    """Run `_wire_turn_agent_callbacks` over minimal fakes; return the agent."""
+    agent = types.SimpleNamespace()
+    ctx = types.SimpleNamespace(
+        progress_callback=None,
+        native_tool_start_callback=None,
+        voice_ack_callback=None,
+        _voice_ack_guild=[None],
+        _native_slack_task_cards=False,
+        native_tool_complete_callback=None,
+        _step_callback_sync=None,
+        _hooks_ref=types.SimpleNamespace(loaded_hooks=[]),
+        _status_callback_sync=None,
+        _event_callback_sync=None,
+        _status_adapter=None,
+        session_key="",
+        user_config=user_config,
+        _thinking_enabled=False,
+        agent_holder=[None],
+        tools_holder=[None],
+        process_task_id=None,
+        process_baseline=None,
+        run_generation=0,
+    )
+    holder = types.SimpleNamespace(
+        _ctx=ctx,
+        _runner=types.SimpleNamespace(
+            _service_tier=None,
+            _consume_pending_turn_sidecar_notes=lambda key: [],
+        ),
+        _make_bg_review_callbacks=lambda: (lambda message: None, lambda: None),
+        _merge_turn_request_overrides=TurnRunner._merge_turn_request_overrides,
+        _clarify_callback_sync=lambda *a, **k: None,
+        _notice_callback_sync=lambda *a, **k: None,
+        _attach_session_title_callback=lambda agent, ctx: None,
+    )
+    TurnRunner._wire_turn_agent_callbacks(holder, agent, {}, None, None, None, False)
+    return agent
+
+
+def test_present_but_null_display_falls_back_to_on():
+    agent = _wire({"display": None})
+    assert agent.memory_notifications == "on"
+
+
+def test_missing_display_falls_back_to_on():
+    agent = _wire({})
+    assert agent.memory_notifications == "on"
+
+
+def test_memory_notifications_setting_still_applies():
+    agent = _wire({"display": {"memory_notifications": "verbose"}})
+    assert agent.memory_notifications == "verbose"