Add validation constraints to field.settings

Problem/Motivation

Field settings have 1 property paths that are not yet validatable:

$ ./vendor/bin/drush config:inspect --filter-keys=field.settings --detail --list-constraints  --fields=key,validatability,constraints
➜  🤖 Analyzing…

 ------------------------------------------- ------------- ------------------------------------------ 
  Key                                         Validatable   Validation constraints                    
 ------------------------------------------- ------------- ------------------------------------------ 
  field.settings                              75%           ValidKeys: '<infer>'                      
                                                            RequiredKeys: '<infer>'                   
   field.settings:                            Validatable   ValidKeys: '<infer>'                      
                                                            RequiredKeys: '<infer>'                   
   field.settings:_core                       Validatable   ValidKeys:                                
                                                              - default_config_hash                   
                                                            RequiredKeys: '<infer>'                   
   field.settings:_core.default_config_hash   Validatable   NotNull: {  }                             
                                                            Regex: '/^[a-zA-Z0-9\-_]+$/'              
                                                            Length: 43                                
                                                            ↣ PrimitiveType: {  }                     
   field.settings:purge_batch_size            NOT           ⚠️  @todo Add validation constraints here  
 ------------------------------------------- ------------- ------------------------------------------ 

Steps to reproduce

1. Get a local git clone of Drupal core 11.x.
2. composer require drupal/config_inspector — or manually install https://www.drupal.org/project/config_inspector/releases/2.1.5 or newer (which supports Drupal 11!)
3. composer require drush/drush
4. vendor/bin/drush config:inspect --filter-keys=olivero.settings --detail --list-constraints

Proposed resolution

Add validation constraints to:

1. purge_batch_size

This requires looking at the existing code and admin UI (if any) to understand which values could be considered valid. Eventually this needs to be reviewed by the relevant subsystem maintainer.

For examples, search *.schema.yml files for the string constraints: 😊

Reach out to @borisson_ or @wimleers in the #distributions-and-recipes.

Remaining tasks

1. purge_batch_size

User interface changes

None.

API changes

None.

Data model changes

More validation 🚀

Release notes snippet

None.