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/background-agent/session-existence.test.ts

@@ -2,12 +2,13 @@ import { describe, expect, mock, test } from "bun:test"
 
 import type { OpencodeClient } from "./opencode-client"
 import { verifySessionExists } from "./session-existence"
+import { unsafeTestValue } from "../../../test-support/unsafe-test-value"
 
 describe("verifySessionExists", () => {
   test("passes query directory to session lookup when provided", async () => {
     // given
     const get = mock(async () => ({ data: { id: "session-123" } }))
-    const client = testCoerce<OpencodeClient>({
+    const client = unsafeTestValue<OpencodeClient>({
       session: {
         get,
       },