refactor: extract shared test fixtures from workflow persistence tests

## Summary

`useWorkflowPersistenceV2.test.ts` and the V1 persistence test file share nearly identical mock setup (settings, toast, i18n, workflowService, app, api, etc.). Extract shared test fixtures to reduce boilerplate.

## Context

Raised during review of #9318 by @christian-byrne.

## Scope

- Extract common mock declarations (settingStore, toast, i18n, workflowService, app graph/api mocks) into a shared test helper
- Update both V1 and V2 persistence test files to use the shared fixtures

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