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>

test-support/unsafe-test-value.ts

@@ -0,0 +1,5 @@
+export function unsafeTestValue<TValue extends PropertyKey>(value: TValue): TValue
+export function unsafeTestValue<TValue = any>(value: unknown): TValue
+export function unsafeTestValue<TValue = any>(value: unknown): TValue {
+  return value as TValue
+}