refactor: create rename-aware detach/attach helper in workflowStore to avoid duplicate mutation paths

## Summary

In `src/platform/workflow/management/stores/workflowStore.ts`, the `renameWorkflow` function currently performs inline mutations of `workflowLookup` and `openWorkflowPaths` after the rename operation (introduced in #9345) instead of reusing the existing `detachWorkflow`/`attachWorkflow` helpers:

```ts
delete workflowLookup.value[oldPath]
workflowLookup.value[workflow.path] = workflow
const openIndex = openWorkflowPaths.value.indexOf(oldPath)
if (openIndex !== -1) {
openWorkflowPaths.value.splice(openIndex, 1, workflow.path)
}
```

This creates a second mutation path for the same data structures, so any future changes to lookup/open-paths logic need to be updated in two places.

## Suggested Follow-up

Consider introducing a rename-aware helper (e.g., `reattachWorkflow(workflow, oldPath, openIndex)`) or adapting the existing `detachWorkflow`/`attachWorkflow` helpers to accept an old-path argument, so that `renameWorkflow` can delegate through the same single code path.

## Context

- Raised by @christian-byrne during review of PR #9345
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/9345#discussion_r2886742393
- Requested by @DrJKL

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