test: make unsafe test coercion explicit

Move test coercion out of a hidden global and require each test to import the helper so review tools and runtime scripts can see the unsafe boundary.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/team-mode/team-runtime/cleanup-team-run-resources.test.ts

@@ -16,6 +16,7 @@ import {
 import { saveRuntimeState } from "../team-state-store/store"
 import type { RuntimeState } from "../types"
 import { cleanupTeamRunResources } from "./cleanup-team-run-resources"
+import { unsafeTestValue } from "../../../../test-support/unsafe-test-value"
 
 const temporaryDirectories: string[] = []
 
@@ -41,7 +42,7 @@ function createRuntimeState(teamRunId: string): RuntimeState {
 }
 
 function createStubBgMgr(): BackgroundManager {
-  return testCoerce<BackgroundManager>({
+  return unsafeTestValue<BackgroundManager>({
     cancelTask: async () => undefined,
   })
 }