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.

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## 1.0.2(2026-07-30)
 
-1.0.1 的续作:那一版把三条跨字段守卫收进构造期后,独立验证发现 `from_env` 上还留着同一类的 15 条校验,一并收拢。
+1.0.1 的续作:那一版把三条跨字段守卫收进构造期后,独立验证发现 `from_env` 上还留着同一类的 15 条校验与 4 条规范化,一并收拢。
 
 ### 修复
 
@@ -18,6 +18,8 @@
 
 同 1.0.1:经 `from_env()` 装配的调用方**不受影响**。手工构造 `GatewaySettings` 或对它 `dataclasses.replace` 的调用方,若配置组合非法,现在会在构造期抛 `ValueError` 并点出字段名,而不是留到运行时表现为静默不建后端、裸 `AssertionError` 或第三方库的天书报错。
 
+**一处静默改值需要留意**:此前手工构造传 `scope="LLM"`(非全小写)的调用方,升级后 scope 会被规范化为 `llm`,**Redis key 随之从 `pgw:limit:LLM:…` 切到 `pgw:limit:llm:…`**。这正是本次要修的问题——旧行为下这批 key 与 `from_env` 装配的进程根本不在同一命名空间;但切换发生的那一刻,旧键上的在途租约会被遗弃,靠 TTL 自愈。滚动升级期间建议留意限流配额短暂偏松。
+
 ## 1.0.1(2026-07-30)
 
 ### 修复