chore: add tsx as devDependency for reliable test subprocess spawning

Author: lewisnsmith

package-lock.json

@@ -50,6 +50,7 @@
     "@types/node": "^25.3.3",
     "eslint": "^10.0.2",
     "tsup": "^8.5.1",
+    "tsx": "^4.21.0",
     "typescript": "^5.9.3",
     "typescript-eslint": "^8.56.1",
     "vitest": "^4.0.18"