docs: document UUID injection behavior in createTemporary() and createNewTemporary()

## Summary

`createTemporary()` and `createNewTemporary()` in `src/platform/workflow/management/stores/workflowStore.ts` now silently inject a UUID `id` field into workflow data via `ensureWorkflowId()`. This was introduced as part of the workflow sharing feature (PR #8951) to give workflows a stable identity for sharing.

## Why this matters

Extensions or consumers that rely on the serialized workflow content matching their original input may silently break, since the returned workflow content will now always contain an `id` field even if none was provided.

## Follow-up tasks

- Add JSDoc to `createTemporary()`, `createNewTemporary()`, and `ensureWorkflowId()` documenting the UUID injection behavior.
- Consider adding a note in the changelog or migration guide for extension authors.
- Evaluate whether to expose this behavior in any public-facing API docs.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/8951
- Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/8951#discussion_r2887572236
- Requested by: @christian-byrne

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-9448-docs-document-UUID-injection-behavior-in-createTemporary-and-createNewTemporary-31a6d73d36508146ad0febc9a6d80c9b) by [Unito](https://www.unito.io)