fix: address completion verdict review findings

- fidelity cacheability also requires a non-empty projected body, so
  think-tag-only answers are no longer cached
- a no-argument tool sent with arguments null only is a deterministic
  dialect: ResultInvalid tool_arguments_null instead of retried
- text mode treats choice index null as 0 and no longer lets False pose
  as 0
- null-only tool_calls elements no longer produce an empty tool_calls
  list; non-stream fidelity checks role before the verdict; warning
  paths are length-bounded
- docs: ARCHITECTURE 7.8 field counts, README cache/watchdog wording,
  CHANGELOG notes, review record in the design, hook note in the plan

Red: 8 new tests failed before this change (e.g.
test_openai_cacheable_requires_projected_body,
test_null_arguments_tool_is_result_invalid_stream,
test_text_mode_choice_index_null_is_zero[None-True]).

tests/unit/test_openai_completion.py

@@ -421,3 +421,48 @@ def test_nonstream_null_tool_elements_stripped():
     assert parse_chat_completion(body).message["tool_calls"] == [tool]
     body["choices"][0]["message"]["tool_calls"] = None
     assert parse_chat_completion(body).message["tool_calls"] is None
+
+
+@pytest.mark.parametrize(("index", "taken"), [(None, True), (0, True), (False, False), (1, False)])
+def test_text_mode_choice_index_null_is_zero(index, taken):
+    """审查 I3: 文本模式 index:null 视缺省(=0);bool 不冒充 0。"""
+    state = ChatCompletionAccumulator(response_mode="text")
+    delta = state.add_chunk(
+        {"choices": [{"index": index, "delta": {"content": "hi"}, "finish_reason": "stop"}]}
+    )
+    assert (delta.content == "hi") is taken and (state.finish_reason == "stop") is taken
+
+
+def test_null_only_tool_elements_do_not_create_empty_list():
+    """审查 Minor1: tool_calls null 后 [null] 不得产出会被上游拒收的空数组。"""
+    state = ChatCompletionAccumulator(response_mode="openai")
+    _feed(
+        state,
+        _frame({"role": "assistant", "content": "a", "tool_calls": None}),
+        _frame({"tool_calls": [None]}, finish="stop"),
+    )
+    assert state.finish().message["tool_calls"] is None
+    body = {"choices": [{"message": {"role": "assistant", "content": "a", "tool_calls": [None]}}]}
+    assert parse_chat_completion(body).message["tool_calls"] is None
+
+
+def test_tool_calls_null_then_list_merges():
+    """tool_calls 先 null 后列表: 正常聚合工具。"""
+    state = ChatCompletionAccumulator(response_mode="openai")
+    tool = {"index": 0, "id": "a", "type": "function", "function": {"name": "f", "arguments": "{}"}}
+    _feed(
+        state,
+        _frame({"role": "assistant", "tool_calls": None}),
+        _frame({"tool_calls": [tool]}, finish="tool_calls"),
+    )
+    assert state.finish().message["tool_calls"][0]["id"] == "a"
+
+
+def test_nonstream_role_checked_before_verdict():
+    """审查 Minor3: 非流式缺 role 是形态失败,先于结局判定。"""
+    from polygateway.transports._openai_completion import check_completion_body
+
+    with pytest.raises(TransientError, match="role"):
+        check_completion_body(
+            {"choices": [{"message": {"content": "a"}, "finish_reason": "length"}]}
+        )