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/stop-continuation-guard/index.test.ts

@@ -6,6 +6,7 @@ import type { PluginInput } from "@opencode-ai/plugin"
 import type { BackgroundManager, BackgroundTask } from "../../features/background-agent"
 import { readContinuationMarker } from "../../features/run-continuation-state"
 import { createStopContinuationGuardHook } from "./index"
+import { unsafeTestValue } from "../../../test-support/unsafe-test-value"
 
 type CancelCall = {
   taskId: string
@@ -31,7 +32,7 @@ describe("stop-continuation-guard", () => {
   })
 
   function createMockPluginInput() {
-    return testCoerce<PluginInput>({
+    return unsafeTestValue<PluginInput>({
       client: {
         tui: {
           showToast: async () => ({}),