Search: purchase "free" from the main plugin redirects to the wrong place

### Impacted plugin

Jetpack

### Steps to Reproduce

1. Start up a free site with just the Jetpack plugin and configure the store sandbox.
2. Go directly to the checkout for a Free plan with something like https://wordpress.com/checkout/established-wildfowl.jurassic.ninja/jetpack_search_free?site=established-wildfowl.jurassic.ninja (You can't click to here until #27003 is fixed).
3. Purchase the free plan

### A clear and concise description of what you expected to happen.

You will get redirected to a link like https://wordpress.com/plans/my-plan/established-wildfowl.jurassic.ninja?thank-you=true&install=all

This is the onboarding for a Free plan on the main plugin, not the onboarding flow for Search Free.

### What actually happened

Should be redirected to https://established-wildfowl.jurassic.ninja/wp-admin/admin.php?page=jetpack-search I think.

I guess this could just be a bug in Calypso.

### Browser

_No response_

### Other information

_No response_

### Platform (Simple, Atomic, or both?)

_No response_

### Reproducibility

Consistent

### Severity

_No response_

### Available workarounds?

_No response_

### Workaround details

_No response_

Reproduce the free Search checkout from a Jetpack-only site using the linked checkout URL, then follow the post-purchase redirect through Calypso and the Jetpack admin page. Done means the purchase lands at wp-admin/admin.php?page=jetpack-search instead of the general Free plan onboarding flow.

Written by the indexing model from the issue text.