fix: consolidate remaining assembly validation into GatewaySettings

Round two of the from_env-only validation problem. Fifteen checks still lived
in the env parsing functions: six enum domains, the redis_url requirement for
redis-backed limiter/breaker/cache, cache namespace and TTL, telemetry path and
DSN, non-negative structured retries and non-blank scope. from_settings and
direct construction bypassed all of them.

The five asserts in client.py that claimed config had already validated
redis_url and the telemetry targets now hold on every path, so they revert to
what CLAUDE.md permits: internal invariant declarations that also narrow the
Optional for type checkers. Their comments now name the method that guarantees
them, since the previous wording is exactly what went stale.

Postgres DSNs built by hand now get the SQLAlchemy +driver suffix stripped the
way from_env has always stripped it, with a warning so the rewrite is not
silent. The env path strips earlier, so it stays quiet.

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+## 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。
+
+### 修复
+
+- **后端选择与条件必填项在任何构造路径上都校验。** 以下此前只有 `from_env` 拦得住,`from_settings()` 与直接构造一律放行:`limiter_backend`/`breaker_backend`/`cache_backend`/`telemetry_backend`/`selector`/`quota_full` 六个字段的合法域;取 `redis` 的后端必须有 `redis_url`;启用缓存必须有 `cache_namespace` 与正 `cache_ttl_s`;`telemetry_backend` 取 `sqlite`/`postgres` 时对应的路径/DSN 必填;`structured_max_retries` 非负;`scope` 非空。
+- **`client.py` 五处断言的前提现在真的成立。** `assert settings.redis_url is not None  # 内部不变量: config 已校验` 之类的注释此前在 `from_settings` 路上是假的:断言开启时抛不含任何字段信息的 `AssertionError`,`python -O` 下断言被移除、错误退化为 redis 库抛出的连接串解析异常。注释已改为点明由哪个校验方法保证。
+- **手工构造时的 Postgres DSN 会剥掉 SQLAlchemy 驱动后缀。** `postgresql+asyncpg://…` 中的 `+asyncpg` asyncpg 不认;`from_env` 一直会剥,直接构造那条路此前不剥,DSN 会一路带到首次写遥测时才炸。现在两条路产出一致,且构造期剥的时候会发一条 warning——库动了调用方给的值,不该静默。经 `from_env` 装配不受影响也不会有这条 warning。
+
+### 行为收紧(下游请读)
+
+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。
+
 ## 1.0.1(2026-07-30)
 
 ### 修复