fix: typed ChatNotFoundError replaces string-prefix sniff in skip routes (T81)

tests/test_drawer_events_threads_skip.py

@@ -273,6 +273,43 @@ def test_post_skip_elision_advances_clock_and_emits_narration(client, tmp_path):
         assert tp["speaker_id"] == "bot_a"
 
 
+def test_skip_route_404_via_typed_exception_class(client, tmp_path):
+    """T81: drawer skip routes 404 via :class:`ChatNotFoundError`.
+
+    Pre-T81, the route caught ``ValueError`` and recovered the 404 case
+    by sniffing ``str(exc).startswith("chat not found")`` — fragile if
+    the message ever changed wording. The controller now raises a typed
+    exception so the route dispatches on type. Asserting the 404 from
+    the unseeded chat exercises the typed branch end-to-end; importing
+    the class confirms it's a real subclass of ``Exception`` and not a
+    re-export of ``ValueError`` (which would defeat the type split).
+    """
+    # Don't seed any chat — the controller hits ``get_chat`` returning
+    # ``None`` and raises ``ChatNotFoundError``. The drawer route then
+    # maps that to ``404`` via the typed handler (no string sniff).
+    _override_llm([])
+    try:
+        response = client.post(
+            "/chats/nonexistent/drawer/skip/elision",
+            data={
+                "landing_state_hint": "x",
+                "new_time": "2026-04-26T20:30:00+00:00",
+            },
+        )
+        assert response.status_code == 404
+    finally:
+        app.dependency_overrides.clear()
+
+    # The exception class itself is importable, distinct from ValueError,
+    # and a proper Exception subclass — pinning the type-based dispatch
+    # so future refactors can't quietly collapse it back to a string sniff.
+    from chat.web.skip import ChatNotFoundError
+
+    assert ChatNotFoundError is not None
+    assert issubclass(ChatNotFoundError, Exception)
+    assert not issubclass(ChatNotFoundError, ValueError)
+
+
 def test_post_skip_elision_invalid_time_returns_400(client, tmp_path):
     _seed_chat(tmp_path / "test.db")
     _override_llm([])