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_client.py

@@ -81,7 +81,7 @@ def _client(sources=None, handler=None, *, limiter=None, quota_full="wait", **ov
             sources={s.name: s for s in sources},
             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": transport,
         "retry": RetryPolicy(3, 2.0, 30.0),
         "backpressure": BackpressurePolicy(300.0, 0.01),