`postHogAdapter` expects `NEXT_PUBLIC_POSTHOG_KEY` but Vercel integration provides `NEXT_PUBLIC_POSTHOG_PROJECT_TOKEN`

[git-johnson]

The default postHogAdapter singleton in @flags-sdk/posthog calls assertEnv("NEXT_PUBLIC_POSTHOG_KEY")

flags/packages/adapter-posthog/src/index.ts

Line 113 in f4b2ec2

postHogKey: assertEnv('NEXT_PUBLIC_POSTHOG_KEY'),

, but the Vercel PostHog integration provisions NEXT_PUBLIC_POSTHOG_PROJECT_TOKEN. This means installing both the integration and using the Flags SDK in your project doesn't produce a working setup. Flags silently fail to evaluate.

I was able to workaround this using the createPostHogAdapter explicitly instead of letting the default singleton do its job.

import { createPostHogAdapter } from "@flags-sdk/posthog";                                 
                                                                                           
  const postHogAdapter = createPostHogAdapter({                                              
    postHogKey: process.env.NEXT_PUBLIC_POSTHOG_PROJECT_TOKEN!,
    postHogOptions: {                                                                        
      host: process.env.NEXT_PUBLIC_POSTHOG_HOST ?? "https://us.posthog.com",              
    },                                                                                       
  });

I suggest the SDK try NEXT_PUBLIC_POSTHOG_PROJECT_TOKEN when NEXT_PUBLIC_POSTHOG_KEY is not set as it would provide the most seamless experience for implementers. However, this does make assumptions that the PostHog integration won't change on you in the future.

[git-johnson]

Please note, I'm happy to open a PR with this change if the suggestion feels sound.