fix(test): narrow mock.module() targets in background-agent tests to prevent barrel corruption

Three test files were mocking the entire '../../shared' barrel, which
corrupted exports for subsequent test files in the same batch run.
Narrow mocks to specific submodules (logger, connected-providers-cache).

Also reverts Bun version pin since the root cause was mock scope, not Bun.

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

@@ -1,7 +1,10 @@
 import { afterAll, beforeEach, describe, expect, mock, test } from "bun:test"
 
-mock.module("../../shared", () => ({
+mock.module("../../shared/logger", () => ({
   log: mock(() => {}),
+}))
+
+mock.module("../../shared/connected-providers-cache", () => ({
   readConnectedProvidersCache: mock(() => null),
   readProviderModelsCache: mock(() => null),
 }))