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/compaction-aware-message-resolver.test.ts

@@ -12,6 +12,7 @@ import {
   setCompactionAgentConfigCheckpoint,
 } from "../../shared/compaction-agent-config-checkpoint"
 import { getCompactionPartStorageDir } from "../../shared/compaction-marker"
+import { unsafeTestValue } from "../../../test-support/unsafe-test-value"
 
 describe("isCompactionAgent", () => {
   describe("#given agent name variations", () => {
@@ -49,7 +50,7 @@ describe("isCompactionAgent", () => {
 
     test("returns false for null", () => {
       // when
-      const result = isCompactionAgent(testCoerce<string>(null))
+      const result = isCompactionAgent(unsafeTestValue<string>(null))
 
       // then
       expect(result).toBe(false)