fix: 装配切换审查修复（启动异常路径韧性/清扫隔离/文案与夹具卫生）

- main.lifespan: 预热+清扫挪进 try,finally dispose_all 覆盖启动半途
  失败(幂等于更少引擎,堵 engine 泄漏);清扫循环体 per-tenant
  try/except + logger.exception 后继续——一个租户坏库不炸平台启动,
  resolve/load_from_dir 保持 fail-fast
- routes/runtime.py: RuntimeError 文案补 registry_reload dispose 旧
  runtime 成因;TenantRuntime 注解挪 TYPE_CHECKING(engine 包运行期
  零 import platform)
- conftest client: teardown 置空 app.state.registry(关残留窗);
  rmtree 先正常删,失败 warnings.warn 留痕后再 ignore_errors 兜底

Co-Authored-By: Claude Code <noreply@anthropic.com>

app/engine/routes/runtime.py

@@ -9,12 +9,17 @@ Task 7 的 Host 中间件上线前,没有人为请求挂 ``state.tenant``。本
 热切换后 default_runtime 仍指向旧 runtime——过渡期不修,Task 7 中间件
 按请求 Host 重新 resolve 后本模块整体删除。
 """
+from typing import TYPE_CHECKING
+
 from fastapi import Request, WebSocket
 
-from app.platform.registry import TenantRuntime
+if TYPE_CHECKING:
+    # 仅类型注解用: engine 包运行期零 import platform(依赖方向保持
+    # platform → engine,反向只在类型层)
+    from app.platform.registry import TenantRuntime
 
 
-def get_runtime(request_or_ws: Request | WebSocket) -> TenantRuntime:
+def get_runtime(request_or_ws: Request | WebSocket) -> "TenantRuntime":
     """state.tenant 优先,缺省落 app.state.default_runtime(Task 7 删)。"""
     runtime = getattr(request_or_ws.state, "tenant", None)
     if runtime is not None:
@@ -22,5 +27,6 @@ def get_runtime(request_or_ws: Request | WebSocket) -> TenantRuntime:
     runtime = getattr(request_or_ws.app.state, "default_runtime", None)
     if runtime is None or runtime.session_factory is None:
         raise RuntimeError(
-            "默认租户运行时未就绪——lifespan 未运行或未预热 default_runtime")
+            "默认租户运行时未就绪——lifespan 未运行、未预热 default_runtime,"
+            "或 registry_reload 已 dispose 旧 runtime(过渡期限制,Task 7 解决)")
     return runtime