PayPal Payment Buttons: reject a return URL longer than PayPal accepts

[millerf]

Fixes N/A

Stacked on #52553 (WOOPTP-510), itself on #52550, #52543 and #52534: the base is add/wooptp-510-changes-made-dialog, and this diff is the last commit only. Retarget to trunk once those merge.

Proposed changes

• PayPal rejects a return URL over 127 characters, and the block passed any length through. The URL Redirect field in the block's form now says "Return URL must be 127 characters or fewer." under a longer one, on top of the existing HTTPS check.
• The server-side attribute validation rejects the same case with a return_url_too_long error, so a longer URL never reaches PayPal whichever way the request arrives.
• The form's return URL error stays advisory, as it has always been: it warns, and the post still saves. The save then fails on the server-side check with the same message in the snackbar, instead of failing at PayPal with a less specific one.

Related product discussion/links

• WOOPTP-482
• PayPal Payment Buttons: ask before leaving a payment link's form with unsaved changes #52553 (WOOPTP-510), the base branch

Does this pull request change what data or activity we track or use?

No.

Testing instructions

The block is behind the paypal-payments-api-managed-buttons feature flag, and needs a site connected to a PayPal (sandbox) account.

• Add or edit a PayPal Payment Button block, open the URL Redirect panel and paste an HTTPS URL longer than 127 characters, for example a post editor URL with its query string. Leave the field: it shows "Return URL must be 127 characters or fewer."
• Shorten it to 127 characters or fewer: the error goes away.
• Save the post with the long URL in place: the snackbar reports that PayPal did not save the button, with the same reason.
• Run jp test js packages/paypal-payments and jp test php packages/paypal-payments.

🤖 Generated with Claude Code

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[jp-launch-control]

Code Coverage Summary

No summary data is available for parent commit d2a02de, so cannot calculate coverage changes. 😴

If that commit is a feature branch rather than a trunk commit, this is expected. Otherwise, this should be updated once coverage for d2a02de is available.

Full summary · PHP report · JS report

[tx2pnw]

LGTM and worked for me.

The merge-base changed after approval.