Allow configuring the batch progress page title per operation

Problem/Motivation

When a Views Bulk Operations action is executed, Drupal displays a batch processing progress page. The title shown on that page is currently hardcoded to:

Performing @operation on selected entities.

While VBO already allows overriding the operation label (shown in the action selector) and the completion message (shown after processing), there is no way to customize the title of the batch progress page itself. This means the title will always read, for example, "Performing Delete entity on selected entities." which might be inconsistent with site language or tone.

Steps to reproduce

1. Install the Views Bulk Operations module and configure a view with a VBO bulk form.
2. Add any bulk operation (e.g. "Delete entity") to the field configuration.
3. Select one or more items in the view and execute the operation.
4. Observe the batch progress page title: it always reads "Performing [operation label] on selected entities." with no way to change it through the UI or configuration.

Proposed resolution

Add an optional "Override batch progress title" field to each operation's configuration settings in the VBO bulk form field configuration UI, consistent with the existing "Override label" and "Override finish message" fields already present. When left empty, the current default title should continue to be used as a fallback.

User interface changes

A new optional text field labeled "Override batch progress title" would appear in the per-operation configuration section of the VBO bulk form field settings, alongside the existing label and message override fields.

API changes

None.

Data model changes

A new batch_title_override key would be added to the VBO configuration data type schema (views_bulk_operations.data_types.schema.yml).