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/plugin/hooks/create-session-hooks.test.ts

@@ -3,8 +3,9 @@ import type { OhMyOpenCodeConfig } from "../../config"
 import type { ModelCacheState } from "../../plugin-state"
 import type { PluginContext } from "../types"
 import { createSessionHooks } from "./create-session-hooks"
+import { unsafeTestValue } from "../../../test-support/unsafe-test-value"
 
-const mockContext = testCoerce<PluginContext>({
+const mockContext = unsafeTestValue<PluginContext>({
   directory: "/tmp",
   client: {
     tui: {