test: add navigation assertion for Locate button in missing-nodes subgraph test

## Summary

The E2E test `Locate node button is visible for expanded pack nodes` in `browser_tests/tests/propertiesPanel/errorsTabMissingNodes.spec.ts` currently only asserts that the Locate button is **visible** after expanding a missing-node pack group. It does not assert that clicking the Locate button actually **navigates** to (focuses) the missing node.

## Reason for deferral

`collectMissingNodes` (called in `loadGraphData`/`app.ts`) builds execution IDs from the original JSON node IDs **before** `rootGraph.configure()` runs. `rootGraph.configure()` performs subgraph node ID deduplication, which can remap colliding node IDs. The pre-configure execution IDs (e.g., `"2:1"`) then mismatch runtime node IDs (e.g., `"2:3"`), causing `focusNode → getNodeByExecutionId` to fail on initial workflow load.

This is tracked and will be fixed in: #10847

## Follow-up work

Once #10847 is resolved, update the test to:
1. Click the Locate button for a missing node inside a subgraph.
2. Assert the canvas/graph has navigated to (focused) the expected node.

Remove the `// TODO` comment in `browser_tests/tests/propertiesPanel/errorsTabMissingNodes.spec.ts` when the assertion is added.

## References

- PR introducing the TODO: #10848 (comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10848#discussion_r3047378050)
- Underlying bug issue: #10847
- Related PRs mentioned in TODO: #9510, #8762

Requested by @DrJKL

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10945-test-add-navigation-assertion-for-Locate-button-in-missing-nodes-subgraph-test-33b6d73d3650817f8de7df56340d4d05) by [Unito](https://www.unito.io)