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/tools/background-task/create-background-task.test.ts

@@ -4,6 +4,7 @@ import { describe, test, expect, mock } from "bun:test"
 import type { BackgroundManager } from "../../features/background-agent"
 import type { PluginInput } from "@opencode-ai/plugin"
 import { createBackgroundTask } from "./create-background-task"
+import { unsafeTestValue } from "../../../test-support/unsafe-test-value"
 
 describe("createBackgroundTask", () => {
   const launchMock = mock(async (): Promise<{
@@ -21,12 +22,12 @@ describe("createBackgroundTask", () => {
   }))
   const getTaskMock = mock()
 
-  const mockManager = testCoerce<BackgroundManager>({
+  const mockManager = unsafeTestValue<BackgroundManager>({
     launch: launchMock,
     getTask: getTaskMock,
   })
 
-  const mockClient = testCoerce<PluginInput["client"]>({
+  const mockClient = unsafeTestValue<PluginInput["client"]>({
     session: {
       messages: mock(() => Promise.resolve({ data: [] })),
     },