Sections-editor tab requires page refresh on first open (sandboxMap race)

[viktormarinho]

Summary

When the user opens the CMS tab from the preview surface before the sandbox pod has finished booting and written its `previewUrl` into the inset entity's `sandboxMap`, the SectionsEditor side panel never mounts. A hard page refresh resolves it because the entity gets re-fetched and the map is populated.

Where it lives

`apps/mesh/src/web/components/sandbox/preview/preview.tsx:808` — the SectionsEditor is gated on:

```tsx
{sectionsOpen && previewUrl && branch && virtualMcpId && (...)}
```

`previewUrl` is derived from `inset?.entity?.metadata?.sandboxMap[userId][branch]`. On the first open of a branch / VM, that slot can be empty for a few seconds while the pod is booting.

Likely fix

When `useSandboxEvents()`'s `lifecycle.phase` transitions to `running` (or specifically when a fresh `sandboxMap` entry is written to the entity), invalidate the React-Query key feeding `useInsetContext` so `previewUrl` re-resolves without a manual page refresh.

Repro

1. Open a virtual MCP from a cold start (no recently-booted pod).
2. Immediately switch the preview to the CMS tab.
3. SectionsEditor panel area shows the loading spinner (or nothing) and stays there.
4. Hard refresh → editor mounts correctly.

Scope note

Surfaced while testing the sections-editor file picker work (#3469). Filing as a follow-up so that PR can stay focused on the picker.