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.

src/polygateway/client.py

@@ -62,7 +62,7 @@ class GatewayClient:
         sources: list[SourceConfig],
         selector: SourceSelector,
         limiter: RateLimiter,
-        gate: ProviderGate,
+        breaker: ProviderGate,
         transport: Transport,
         retry: RetryPolicy,
         backpressure: BackpressurePolicy,
@@ -84,7 +84,7 @@ class GatewayClient:
             sources=sources,
             selector=selector,
             limiter=limiter,
-            gate=gate,
+            gate=breaker,
             transport=transport,
             retry=retry,
             backpressure=backpressure,
@@ -183,7 +183,7 @@ class GatewayClient:
         settings: GatewaySettings,
         *,
         limiter: RateLimiter | None = None,
-        gate: ProviderGate | None = None,
+        breaker: ProviderGate | None = None,
         cache: CacheBackend | None = None,
         telemetry: TelemetryRecorder | None = None,
         registry: Mapping[str, ProviderProfile] | None = None,
@@ -203,7 +203,7 @@ class GatewayClient:
                 global_limits=settings.global_limits,
                 lease_ttl_s=settings.lease_ttl_s,
             ),
-            gate=gate or InMemoryGate(config=settings.breaker),
+            breaker=breaker or InMemoryGate(config=settings.breaker),
             transport=OpenAICompatTransport(registry=registry),
             retry=settings.retry,
             backpressure=settings.backpressure,
@@ -223,7 +223,7 @@ class GatewayClient:
         scope: str = "LLM",
         *,
         limiter: RateLimiter | None = None,
-        gate: ProviderGate | None = None,
+        breaker: ProviderGate | None = None,
         cache: CacheBackend | None = None,
         telemetry: TelemetryRecorder | None = None,
         registry: Mapping[str, ProviderProfile] | None = None,
@@ -233,7 +233,7 @@ class GatewayClient:
         return cls.from_settings(
             GatewaySettings.from_env(scope, env=env),
             limiter=limiter,
-            gate=gate,
+            breaker=breaker,
             cache=cache,
             telemetry=telemetry,
             registry=registry,