fix: address independent verification findings

Classify empty completions as transient per human ruling (fixes flaky
real-gateway smoke and prevents caching empty responses), rename the
factory injection parameter gate to breaker per the frozen design,
rewrite the probe-entry cleanup without except BaseException, declare
python-dotenv explicitly, add a mid-backoff cancellation test, and
record all implementation errata in the design and architecture docs.

tests/unit/test_openai_compat.py

@@ -161,6 +161,33 @@ class TestMissingDoneSemantics:
             await _complete(_transport_for(handler), _source(missing_done="salvage"))
 
 
+class TestEmptyCompletion:
+    """空补全 → TransientError(2026-07-20 人类裁决;MiniMax 间歇形态,绝不缓存)。"""
+
+    async def test_stream_zero_content_with_done_is_transient(self):
+        def handler(request):
+            return _sse_stream(_chunk(reasoning="only thinking"), _chunk(usage=_USAGE))
+
+        with pytest.raises(TransientError, match="empty_completion"):
+            await _complete(_transport_for(handler), _source())
+
+    async def test_non_stream_empty_content_is_transient(self):
+        def handler(request):
+            return httpx.Response(
+                200, json={"choices": [{"message": {"content": ""}}], "usage": _USAGE}
+            )
+
+        with pytest.raises(TransientError, match="empty_completion"):
+            await _complete(_transport_for(handler), _source(), stream=False)
+
+    async def test_think_only_content_after_strip_is_transient(self):
+        def handler(request):
+            return _sse_stream(_chunk(content="<think>hmm</think>"), _chunk(usage=_USAGE))
+
+        with pytest.raises(TransientError, match="empty_completion"):
+            await _complete(_transport_for(handler), _source())
+
+
 class TestNonStreamFastPath:
     async def test_non_stream_parses_message(self):
         def handler(request):