fix: make h2 patch repair failures diagnosable

packages/casan-harness/tests/phase3-model-router-tests.sh

@@ -150,6 +150,15 @@ class FakeResp:
     def read(self):
         return json.dumps(self.payload).encode()
 
+class EmptyResp:
+    headers = {"Content-Type": "application/json"}
+    def __enter__(self):
+        return self
+    def __exit__(self, exc_type, exc, tb):
+        return False
+    def read(self):
+        return b""
+
 def fake_urlopen(req, timeout):
     seen.append((req.full_url, dict(req.header_items()), json.loads(req.data.decode())))
     if req.full_url.endswith("/v1/responses"):
@@ -207,6 +216,14 @@ for fn, bad in (
         assert exc.code == 2, exc.code
     else:
         raise AssertionError(f"{fn.__name__} accepted malformed provider payload")
+
+try:
+    with contextlib.redirect_stderr(io.StringIO()):
+        mc.decode_provider_json(EmptyResp(), "openai-compatible")
+except SystemExit as exc:
+    assert exc.code == 2, exc.code
+else:
+    raise AssertionError("empty provider response was accepted")
 print("ok")
 PY
 [[ $? -eq 0 ]] && pass "cloud/gateway provider responses parse real usage and reject malformed payloads" || fail "cloud/gateway provider parser coverage failed"