Add configuration schema for views_pdf.settings

Problem/Motivation

After installing the module and running drush config:inspect --detail --only-error, we get the following error:

-------------------- ----------- ------------- ------ 
  Key                  Status      Validatable   Data  
 -------------------- ----------- ------------- ------ 
  views_pdf.settings   No schema                       
 -------------------- ----------- ------------- ------ 

The module ships config/install/views_pdf.settings.yml, but neither config/schema/views_pdf.schema.yml nor config/schema/views_pdf.views.schema.yml defines a views_pdf.settings config object.

Steps to reproduce

1. On a clean Drupal 11.4 site, run drush config:inspect --detail --only-error. No errors are reported.
2. Install the module: drush en -y views_pdf
3. Run drush config:inspect --detail --only-error again.
4. views_pdf.settings is reported with status “No schema”.

Proposed resolution

Add a views_pdf.settings definition to the existing config/schema/views_pdf.schema.yml