fix: address Cubic findings for runtime fallback child sessions

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

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

src/hooks/runtime-fallback/error-classifier.test.ts

@@ -1,5 +1,4 @@
-declare const require: (name: string) => any
-const { describe, expect, test } = require("bun:test")
+import { describe, expect, test } from "bun:test"
 
 import { classifyErrorType, extractAutoRetrySignal, isRetryableError } from "./error-classifier"