test: add typed test coercion helper

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/testing/test-coerce-global.d.ts

@@ -0,0 +1,5 @@
+declare global {
+	function testCoerce<TValue>(value: TValue): TValue
+}
+
+export {}