Migrating from shop-based to user-based token strategy

Issue summary

Before opening this issue, I have:

• shopify_app version: 21.10

• Ruby version: 3.3.0

• Found a reliable way to reproduce the problem that indicates it's a problem with the package

• Looked for similar issues in this repository

• Checked that this isn't an issue with a Shopify API

I am moving an existing app with existing shops using the app from shop-based token to user-based token
I followed the 3 steps listed here: https://github.com/Shopify/shopify_app/blob/main/docs/shopify_app/sessions.md#migrating-from-shop-based-to-user-based-token-strategy

My belief is that it means I keep shops and users at the same time

When I try to install the app I see multiple OAuth attempts in my logs (goes to '/login?host...' then '/auth/shopify/callback?...' multiple times in a row)
Then the webpage shows an error mentioning too many redirects and I am taken back to Shopify Admin and shown an error message that an error occurred with the app

Expected behavior

App is installed / open / goes through OAuth, and user is created or found in the app db (same as what happens with shops basically)

Actual behavior

Error message, multiple OAuth attempts / redirects (as described above)

Steps to reproduce the problem

1. Take an existing app with shop-based token
2. Follow 3 steps to migrate to user-based token (link above)
3. Open app / install app on store

Debug logs

Start with the migration instructions in docs/shopify_app/sessions.md, then trace the /login and /auth/shopify/callback entry points during the reproduced install. Compare the OAuth redirect sequence with the expected user creation or lookup and verify the flow completes without repeated redirects.