Breaking: #107677 - Drop prepend and append modes from TCA value picker 

See forge#107677

Description 

The prepend and append modes of the value picker specified in TCA and used in FormEngine (for example $GLOBALS['TCA']['tx_example']['columns']['example']['config']['valuePicker']['mode'] ) have been removed.

These modes were designed to insert predefined values before or after the existing input value, but they served only niche use cases and caused inconsistent behavior across different input types.

Maintaining these modes introduced unnecessary complexity in both implementation and accessibility. Prepending or appending content dynamically to user input fields could easily lead to unexpected results, break input validation, and interfere with assistive technologies such as screen readers. Additionally, this approach mixed presentation and data logic in ways that are not consistent with modern form handling patterns.

Future improvements to the value picker will focus on a consistent mode=replace behavior and may be implemented as new form input types to provide a more robust and accessible user experience.

Impact 

Any value picker TCA configuration using the mode options prepend or append will no longer have any effect. TYPO3 ignores these settings, and the picker defaults to the standard replace behavior.

Affected installations 

Installations using custom field wizard configurations or integrations that rely on the value picker with mode = prepend or mode = append are affected.

Migration 

There is no direct replacement for the removed modes.

If your implementation depends on prepending or appending content to existing values, implement a custom input type or custom form element to handle this behavior explicitly. This allows you to maintain full control over the user interface, data handling, and accessibility.

For most use cases, it is recommended to replace prepend or append logic with the standard replace value picker behavior or use dedicated UI controls that clearly indicate how values are modified.

If the value picker was used to suggest combinable values, consider listing these elements in the field description instead so that users can copy and paste them manually into the input field.