test: synchronize ComfyApi WebSocket handler setup

## Summary

`src/scripts/api.featureFlags.test.ts` contains several tests that use this pattern:

```ts
const initPromise = api.init()
// Invoke mocked WebSocket handlers
await initPromise
```

`ComfyApi.init()` returns `void`, so awaiting its return value does not synchronize socket initialization. The tests currently pass because the Vitest distribution is `localhost` and WebSocket handlers are registered synchronously, but the pattern is fragile if socket creation needs to await authentication or other asynchronous setup.

## Scope

- Introduce a small test helper that calls `ComfyApi.init()` and waits until the fresh WebSocket mock has registered both the `open` and `message` handlers.
- Migrate all identical initialization patterns in `api.featureFlags.test.ts` together.
- Keep the public production `ComfyApi.init()` signature unchanged unless there is a separate production requirement to make initialization awaitable.

## Acceptance criteria

- No test treats the return value of `ComfyApi.init()` as a promise.
- Tests dispatch mocked WebSocket events only after the current mock has registered the required handlers.
- Feature flag and server-message tests continue to pass.
- Coverage includes an asynchronously delayed socket-handler registration path.

## Context

Raised during review of #14041: https://github.com/Comfy-Org/ComfyUI_frontend/pull/14041#discussion_r3681871605