Spike: extract a shared component-render test helper to reduce test harness duplication

## Description

Multiple component test files duplicate the same render-wrapper pattern: create a `QueryClient`, wrap the component in `QueryClientProvider` + jotai `Provider` + `TooltipProvider`, seed state via `DbState.applyTo(store)`. This pattern appears at least in `SchemaExplorerSidebar.test.tsx`, `Styles.test.tsx`, and others with similar shape.

The seam is: a render function that wraps any component in the standard required context providers, given a `DbState` to seed. Today every test file re-derives this boilerplate.

## Questions to investigate

- What is the right abstraction boundary? A `renderWithState(component, state)` that returns the standard `render()` result? Or a reusable wrapper-component factory?
- Should it compose with the existing `renderHookWithState` (which already handles the Jotai store + QueryClient for hook tests)? Is there a unified `TestProvider` that both can share?
- Does the `vi.mock("react-virtuoso", ...)` block (also duplicated across files that render Virtuoso lists) belong in a shared mock utility, or should it stay per-file since mocking has test-file scope?
- What about the `getAppStore()` call — should the helper own creating a fresh store, or should callers still pass one?

## Desired outcome

A concrete recommendation (with a sketch) for a `@/utils/testing` helper that reduces component-test boilerplate without coupling tests to each other. The helper should be opt-in (existing tests don't break if they don't adopt it) and composable with `DbState`.

## Related

- Observed in #1914 (Combine styling sidebars) — `renderStyles` and `renderSidebar` are near-identical.
- `renderHookWithState` already solves the hook-level version of this pattern.

> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.