test: trim fast-status regression test to the invariant cases

Keep '' -> normal (the bug), None -> normal and priority -> fast (passthrough
sanity), both for a built agent and a pre-build pin, plus the 'status never
writes config' guard. Drop the redundant identity rows and raising=False
(both patched names exist on the module).

tests/tui_gateway/test_empty_fast_status.py

@@ -1,21 +1,29 @@
+"""``config.set fast status`` reports "normal" for an explicit-normal pin.
+
+A session pinned to normal carries ``""`` (not ``None``) in ``create_service_tier_override``
+and, once built, in ``agent.service_tier`` — the status branch echoed that ``""`` back.
+"""
+
 from types import SimpleNamespace
+from unittest.mock import patch
+
 import pytest
-from tui_gateway import methods_config_set as config
+
+import tui_gateway.server as server
 
 
-@pytest.mark.parametrize('tier,expected', [('', 'normal'), (None, 'normal'), ('priority', 'fast'), ('normal', 'normal'), ('auto', 'auto'), ('cold', 'cold')])
-@pytest.mark.parametrize('built', [True, False])
-def test_fast_status_preserves_tier_semantics_without_mutation(monkeypatch, tier, expected, built):
-    session = {'agent': SimpleNamespace(service_tier=tier)} if built else {'create_service_tier_override': tier}
-    replies = []
-    monkeypatch.setattr(config, '_load_service_tier', lambda: tier, raising=False)
-    monkeypatch.setattr(config, '_kv', lambda rid, key, value: replies.append((rid, key, value)))
-    def forbidden(*args, **kwargs):
-        pytest.fail('Status query must not write configuration')
-    monkeypatch.setattr(config, '_write_config_key', forbidden, raising=False)
-    config._set_fast(1, {'session_id': 'test'}, 'fast', 'status', session)
-    assert replies == [(1, 'fast', expected)]
+@pytest.mark.parametrize("tier,expected", [("", "normal"), (None, "normal"), ("priority", "fast")])
+@pytest.mark.parametrize("built", [True, False])
+def test_fast_status_reports_tier_without_writing_config(tier, expected, built):
+    session = ({"agent": SimpleNamespace(service_tier=tier)} if built
+               else {"create_service_tier_override": tier})
+    with patch.dict(server._sessions, {"s-fast": session}, clear=False), \
+            patch.object(server, "_load_service_tier", return_value=tier), \
+            patch.object(server, "_write_config_key") as write_key:
+        res = server._methods["config.set"]("rid", {"session_id": "s-fast", "key": "fast", "value": "status"})
+    assert res["result"]["value"] == expected
+    write_key.assert_not_called()
     if built:
-        assert session['agent'].service_tier == tier
+        assert session["agent"].service_tier == tier
     else:
-        assert session['create_service_tier_override'] == tier
+        assert session["create_service_tier_override"] == tier