fix(ci): resolve mock.module() cross-file leakage in test suite

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

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

src/features/background-agent/fallback-retry-handler.test.ts

@@ -17,14 +17,15 @@ mock.module("../../shared/provider-model-id-transform", () => ({
   transformModelForProvider: mock((_provider: string, model: string) => model),
 }))
 
-import { tryFallbackRetry } from "./fallback-retry-handler"
-import { shouldRetryError } from "../../shared/model-error-classifier"
-import { selectFallbackProvider } from "../../shared/model-error-classifier"
-import { readProviderModelsCache } from "../../shared"
 import type { BackgroundTask } from "./types"
 import type { ConcurrencyManager } from "./concurrency"
 import type { OpencodeClient, QueueItem } from "./constants"
 
+const { tryFallbackRetry } = await import("./fallback-retry-handler")
+const { shouldRetryError, selectFallbackProvider } = await import("../../shared/model-error-classifier")
+const { readProviderModelsCache } = await import("../../shared")
+mock.restore()
+
 function createDeferredPromise(): {
   promise: Promise<void>
   resolve: () => void