Feature: #110850 - Try out reactions in the backend module

Description

Each reaction in the Administration > Integrations > Reactions module now has a Try out button. It opens a modal showing what an external system has to send to that reaction, and lets an administrator check a payload against it before any request is sent.

The modal offers:

Endpoint: The absolute URL of the reaction, with a button to copy it.
Example: A curl command for this reaction, with a button to copy it. Its JSON body is composed of the placeholders the field map reads, so ${customer.name} becomes {"customer": {"name": "value"}}. The secret is shown as a placeholder only.
Dry run: Offered for reactions creating a database record. A payload, prefilled with the example body, is edited in a JSON code editor and resolved against the field map. The result lists the fields that would be written and their values, and the fields that would be skipped together with the reason, the same reasons the endpoint reports in skippedFields.

The dry run resolves the payload as the backend user the reaction impersonates, including the item processors of a field, so field permissions and offered items are the ones the endpoint applies. No record is written and the secret is not checked.

It also names what would keep the endpoint from creating any record at all, whatever the payload:

the reaction is disabled, not active yet, or expired,
the reaction impersonates no backend user, or one that is disabled or deleted,
the impersonated user may not modify records of the target table,
the storage page does not exist, or the impersonated user may not create records on it.

The URL and the inline example previously shown in the list are replaced by the Try out button.

Note

The dry run does not repeat every check of DataHandler. Whether the page type of the storage page allows the table, the root level restrictions of a table, and the evaluation of a field value are only known when the record is actually written.

Impact

Integrators configuring a reaction see the request the reaction expects rather than a generic example, and can verify a field map, the permissions of the impersonated user and the storage page before connecting an external system. Mistakes that previously only showed up as a 400 response, a skipped field or a missing record are visible in the backend module.