fix(#20): heid-code-review fixups — INV-CUT-2 completeness on cancel/stream (slice-2)

Triaged the heid-code-review panel (Gróa + Hulda substantive, Regin zero=weak).

Adopted (genuine adds):
- cancel_turn + stream_turn gain a defensive `except ApiError -> SessionApiFailed`
  default after their discriminated branches. INV-CUT-2 ("every ApiError is mapped;
  default SessionApiFailed") now holds STRUCTURALLY on those routes, not by coupling
  to the SDK's internal guarantee that it maps them to discriminated types. + tests.
- get_session_tools error-path test (symmetric with messages).
- Contract § Error map amended: added the stream ProtocolError rows
  (Malformed*/TurnIdFlip -> ratatoskr same-named), clarified the cancel row (the SDK
  RAISES the typed races -> ratatoskr exceptions, only a 200/cancelled=False is a
  CancelResult; caller surface stays exception-based per DEC-2), and noted the
  ApiError default holds on stream+cancel too.

Rejected (category-5, wrong-grounding) — 2/3 arms flagged create's bound-502 as
"should gate on error_code like list's 422+cursor_invalid". Verified against the SDK
parser (not in the arms' file set): the bound-502 body is
{"error_code":"bifrost_handshake_failed","detail":{"bifrost_error":...}}, and the
SDK's envelope parser PREFERS the nested detail (which lacks error_code), so
ApiError.error_code resolves to "unknown" — gating would REGRESS handshake detection
(the cli/web integration tests caught it). INV-002 also makes the handshake the sole
bound-502 cause. Kept the any-bound-502 mapping; documented WHY in code + contract.

Accepted-as-is: create_session -> Mapping annotation (intentional open-world
passthrough, already documented in the route-map note; category 3).

Suite 493 green; wt.py mypy + ruff clean. Patch.

tests/test_wt.py

@@ -299,6 +299,12 @@ class TestReadPassthroughs:
             await get_session_messages(_wt(fake), "s")
         assert ei.value.status == 401
 
+    async def test_tools_error_maps_to_session_api_failed(self) -> None:
+        fake = _FakeSessions(error=ApiError("auth_revoked", "no", status=401))
+        with pytest.raises(SessionApiFailed) as ei:
+            await get_session_tools(_wt(fake), "s")
+        assert ei.value.status == 401
+
 
 async def _drain(aiter: Any) -> list[Any]:
     out: list[Any] = []
@@ -376,6 +382,14 @@ class TestStreamTurn:
             await _drain(stream_turn(_wt(fake), "s", "hi"))
         assert (ei.value.established, ei.value.got) == (5, 7)
 
+    async def test_undiscriminated_api_error_maps_to_session_api_failed(self) -> None:
+        # INV-CUT-2 default: an undiscriminated ApiError surfacing from the stream
+        # (not a discriminated stream error) → SessionApiFailed.
+        fake = _FakeSessions(stream_error=ApiError("weird", "boom", status=500))
+        with pytest.raises(SessionApiFailed) as ei:
+            await _drain(stream_turn(_wt(fake), "s", "hi"))
+        assert ei.value.status == 500
+
 
 class TestCancelTurn:
     async def test_happy_returns_cancel_result(self) -> None:
@@ -410,3 +424,11 @@ class TestCancelTurn:
         fake = _FakeSessions(error=wtsdk.CancelFailed(42, error_code="boom", message="failed"))
         with pytest.raises(CancelFailed):
             await cancel_turn(_wt(fake), "s", 42)
+
+    async def test_undiscriminated_api_error_maps_to_session_api_failed(self) -> None:
+        # INV-CUT-2 default: an undiscriminated ApiError on the cancel route (not a
+        # typed Cancel* race) → SessionApiFailed, never leaked as a bare ApiError.
+        fake = _FakeSessions(error=ApiError("weird", "boom", status=500))
+        with pytest.raises(SessionApiFailed) as ei:
+            await cancel_turn(_wt(fake), "s", 42)
+        assert ei.value.status == 500