[Donations Form Block] One-time donations incorrectly show a 'Next Donation Date' in confirmation email

### Impacted plugin

None / Other

### Quick summary

When making a one-time donation, the email that gets sent out with the subject `You’ve donated to [site title]` shows a `Next Donation Date`, even though the donation is one-time.

This is causing confusion for users who make a one-time donation and think that they may be charged again.

### Steps to reproduce

1. Add [Donations Form Block](https://wordpress.com/support/wordpress-editor/blocks/donations/) to site.
2. Make a one-time donation.
3. The email you receive with the subject `You’ve donated to [site name]` will mention a `Next Donation Date`.

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Minor

### What other impact(s) does this issue have?

No revenue impact

### If a workaround is available, please outline it here.

_No response_

### Platform (Simple and/or Atomic)

Atomic, Simple

Start by reproducing the one-time donation flow described in the issue and inspect the confirmation email it generates. Trace the Donations Form Block’s email implementation to find where Next Donation Date is included. Done means one-time donation confirmations omit that field while recurring donation confirmations retain it, with relevant tests updated or added if the existing test coverage is found.

Written by the indexing model from the issue text.