Update how the fraud prevention token is added to the checkout page

[eduardoumpierre]

Description

There are occurrences where the Fraud Prevention Token is not added to the checkout page due to restrictions in the the_content filter, making the checkout form fail.

p1706826656327619-slack-C03EE4CPPK5 was one of those occurrences, which took a considerable time to understand and debug the real issue.

Suggestions to fix this issue:

• p1707221154160759/1706826656.327619-slack-C03EE4CPPK5 suggests that wp_footer would be a better filter to hook into instead of the_content.
• p1707221337069669/1706826656.327619-slack-C03EE4CPPK5 suggests that the token can be added as a global JS variable.

How to reproduce it

• Enable fraud_prevention_service on your site
• Go to your checkout page, inspect the form, and search for wcpay-fraud-prevention-token
  • It should be present on the page
• Change your theme to any that has Site Editor support – such as "Twenty Twenty Four"
• Go to Appearance > Editor
• On the Site Editor, go to Templates > Page: Checkout > Customize
• On the List View, delete all the blocks that aren't the header or footer
• Add a new checkout block between them
• Save the changes
• Go back to your checkout page, inspect the form, and search for wcpay-fraud-prevention-token
  • It should not be present on the page

Acceptance criteria

• The checkout form works even to pages without "the content" block

Additional context

• Related Slack thread (p1706826656327619-slack-C03EE4CPPK5)

[zmaglica]

This issue impacts Risk & Fraud, so assigning to Sigma (based on team responsibilities Pc2DNy-3z-p2) @dwainm .
Assigning as part of Gamma Triage process PcreKM-yM-p2.