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.

pyproject.toml

@@ -11,6 +11,9 @@ dependencies = [
     "httpx>=0.27",
     "pydantic>=2.8",
     "pydantic-settings>=2.4",
+    # 显式声明(2026-07-20 人类确认): config.py 的多源动态键族直接使用 dotenv_values,
+    # 不能只依赖 pydantic-settings 的传递安装
+    "python-dotenv>=1.0",
     "loguru>=0.7",
 ]