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/integration/test_redis_cache.py

@@ -155,7 +155,7 @@ class TestClientWithRealRedis:
             limiter=InMemoryLimiter(
                 scope="llm", sources={src.name: src}, global_limits=GlobalLimits(0, 0, 0)
             ),
-            gate=InMemoryGate(config=BreakerConfig(5, 60.0, 120.0)),
+            breaker=InMemoryGate(config=BreakerConfig(5, 60.0, 120.0)),
             transport=OpenAICompatTransport(
                 client_factory=lambda s: httpx.AsyncClient(
                     transport=httpx.MockTransport(lambda req: _sse_response())