Errors from another workflow may persist after switching to a drag-dropped API workflow

## Symptom (observed, not yet reproduced)

An API-format workflow opened by **dragging and dropping** the JSON onto the canvas appeared to keep showing a *different* workflow's errors after switching tabs into it — i.e. the incoming tab did not clear the outgoing workflow's error state.

Filing so it is not lost. I could not reproduce it, and the original observation was made on a branch that no longer exists, so the symptom may be stale.

## What was tested

On a local tree with #14555 and #14557 merged, two workflow tabs were created, each with a **different** uninstalled node type, and the error panel and overlay were read at each step:

| step | panel | overlay |
|---|---|---|
| tab-alpha active | `Unknown pack MissingNodeAlpha` | `Missing node: MissingNodeAlpha` |
| tab-beta active | — | `Missing node: MissingNodeBeta` |
| switched back to tab-alpha | — | none |

No leakage: while `tab-beta` was active, nothing from `tab-alpha` was shown. The reported pollution did not occur.

The absent overlay in the last row is by design — tab switching calls `showPendingWarnings(undefined, { silent: true })` from `workflowService.openWorkflow`, and `silent` suppresses `showErrorOverlay()` while still syncing the store.

## What was NOT covered

- **The drag-and-drop entry path.** The test drove `window.app.loadApiJson(...)` directly rather than dropping a file, so anything specific to the drop handler (`deferWarnings: true` for each file, then a single `showPendingWarnings()` after the loop) is untested here.
- The original branch the symptom was seen on. It was a pre-restructure version of the errors work that was discarded, so the behaviour there is not recoverable.

## Repro steps to try

1. Open a workflow that reports an error (missing node or missing model).
2. Drag and drop an API-format JSON referencing an uninstalled node type onto the canvas.
3. Switch between the two tabs and watch the Errors panel.

Expected: each tab shows only its own errors.

Trace the drag-and-drop entry path, comparing it with window.app.loadApiJson(...), then inspect workflowService.openWorkflow and the deferWarnings/showPendingWarnings flow. Done means a drag-dropped API workflow has coverage and switching tabs shows only the active workflow's errors, including the expected silent overlay behavior.

Written by the indexing model from the issue text.