[Bug]: Purchasing Jetpack Search is still possible on jetpack.com even if the site already has a Search subscription

### Quick summary

If you have the Search plugin installed on your Jetpack site with a current Search subscription, then go to Jetpack.com, you can still purchase a new Search plan.

### Steps to reproduce

1. On a test non-WordPress.com site, install the Search plugin: https://wordpress.org/plugins/jetpack-search/
2. From Jetpack > Search, sign up for a new search subscription.
3. Go to https://cloud.jetpack.com/pricing and try to purchase a Site Search plan.
4. Enter the URL of your test site and continue through the purchase process.

### What you expected to happen

If the site URL I enter already has a Jetpack Search plan, I would expect jetpack.com to tell me that and not let me proceed with the purchase.

### What actually happened

I was able to proceed with the purchase on jetpack.com and it added a month to my existing subscription.

### Browser

_No response_

### Context

_No response_

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

_No response_

### Other notes

Noticed by @ederrengifo in p1HpG7-hkA-p2.

### Reproducibility

Consistent

### Severity

_No response_

### Available workarounds?

_No response_

### Workaround details

_No response_

Start by reproducing the flow from the Jetpack Search plugin through cloud.jetpack.com/pricing using a test non-WordPress.com site with an existing Search subscription. Trace the pricing purchase flow and its site-URL subscription check; done means an existing Search plan is detected, the duplicate purchase cannot proceed, and the user is informed.

Written by the indexing model from the issue text.