test: cover the env-key messages left unguarded by mutation testing

Mutation testing showed the negative structured-retries and expected-dim checks
in the env parsing path could be deleted with every test still passing. Their
value is the env key name in the message, so they need tests that assert it.

Changelog now states the real scope of this release and warns that normalising
scope moves the Redis keys, the one change here that silently relocates runtime
state. Records the breaker threshold derivation as deliberately env-only so it
does not resurface as another round.

research-wiki/designs/2026-07-30-settings-invariants-round-2-design.md

@@ -96,6 +96,7 @@ assert settings.redis_url is not None  # 内部不变量: config 已校验
 | `from_env` 对上述 15 条的校验与报错 | **全部保留**,时机提前到 `cls(...)`;`_load_*` 内联检查删除,避免同一约束两处维护 |
 | `_load_pg_dsn` 剥 `+driver` | **保留**,继续只在 env 路径生效(§5) |
 | `_load_choice` 的 `default` 语义(键缺失时取默认) | **保留**,那是 env 解析职责,不是不变量 |
+| `_load_breaker` 的有效阈值派生 `max(配置值, 源级并发×2)` | **有意保留在 env 层**(verifier 二次核验点名,记此备案免成"第五批")。它是**派生**不是校验/规范化:两路产出确实不同(env 装配 threshold=5/并发=100 得 200,直接构造得 5),但派生依赖的是"用户没显式表态时库替他选一个合理值"的 env 语义;代码构造那条路,调用方给什么就是什么表态。其跨字段下限风险由 `_validate_probe` 在构造期兜底 |
 | 直接构造出上述任一非法组合 → 静默成功 | **有意替换**为构造期 `ValueError` |
 | `client.py` 5 处 assert | **保留**,仅改注释(§4) |
 | 异常类型 | 一律 `ValueError`,与第一轮及既有装配错误一致 |