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/hooks/auto-update-checker/checker/cached-version.test.ts

@@ -15,7 +15,7 @@ mock.module("../constants", () => ({
       const current = mockState.candidates
       // Forward array methods/properties to the mutable candidates list
       // so getCachedVersion's `for (... of ...)` sees fresh data per test.
-      const value = (testCoerce<Record<PropertyKey, unknown>>(current))[prop]
+      const value = (unsafeTestValue<Record<PropertyKey, unknown>>(current))[prop]
       if (typeof value === "function") {
         return (value as (...args: unknown[]) => unknown).bind(current)
       }
@@ -29,6 +29,7 @@ mock.module("./package-json-locator", () => ({
 }))
 
 import { getCachedVersion } from "./cached-version"
+import { unsafeTestValue } from "../../../../test-support/unsafe-test-value"
 
 describe("getCachedVersion (GH-3257)", () => {
   let cacheRoot: string