Purchase Data: Always return purchase response once purchase data is sent

[n7studios]

Summary

When custom fields were mapped, send_purchase_data() returned the update subscriber response instead of the purchase data response. Sync Past Orders (REST API and WP-CLI) then:

• reported the Order as failed if updating custom fields failed, even though the purchase data was sent
• output an empty Kit Purchase ID via WP-CLI if updating custom fields succeeded
• received null if no subscriber ID was returned

send_purchase_data() now always returns the purchase data response once the purchase data is sent. The custom fields update result is only used for the Order note.

Testing

• RESTAPITest::testSyncPastOrderWhenCustomFieldsUpdateFails: when updating custom fields fails, the REST API route still returns success with the Kit Purchase ID.
• SyncPastOrdersCLICest::testSyncPastOrdersWithCustomFields: when custom fields are mapped, the CLI output includes the Kit Purchase ID.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)