refactor: centralize test path containment checks

test/helpers/paths.ts

@@ -0,0 +1,19 @@
+import path from "node:path";
+
+export function isPathWithinBase(base: string, target: string): boolean {
+  if (process.platform === "win32") {
+    const normalizedBase = path.win32.normalize(path.win32.resolve(base));
+    const normalizedTarget = path.win32.normalize(path.win32.resolve(target));
+
+    const rel = path.win32.relative(
+      normalizedBase.toLowerCase(),
+      normalizedTarget.toLowerCase(),
+    );
+    return rel === "" || (!rel.startsWith("..") && !path.win32.isAbsolute(rel));
+  }
+
+  const normalizedBase = path.resolve(base);
+  const normalizedTarget = path.resolve(target);
+  const rel = path.relative(normalizedBase, normalizedTarget);
+  return rel === "" || (!rel.startsWith("..") && !path.isAbsolute(rel));
+}