Add validation constraints to system.site

Problem/Motivation

system.site has 5 property path that are not yet validatable:

vendor/bin/drush config:inspect --filter-keys=system.site --detail --list-constraints
➜  🤖 Analyzing…

 Legend for Data: 
  ✅❓  → Correct primitive type, detailed validation impossible.
  ✅✅  → Correct primitive type, passed all validation constraints.
 ---------------------------------------- --------- ------------- ------ --------------------------------------------------------------------------------------------- 
  Key                                      Status    Validatable   Data   Validation constraints                                                                       
 ---------------------------------------- --------- ------------- ------ --------------------------------------------------------------------------------------------- 
  system.site                              Correct   69%           ✅❓   ValidKeys: '<infer>'                                                                         
                                                                          LangcodeRequiredIfTranslatableValues: null                                                   
   system.site:                            Correct   Validatable   ✅✅   ValidKeys: '<infer>'                                                                         
                                                                          LangcodeRequiredIfTranslatableValues: null                                                   
   system.site:_core                       Correct   Validatable   ✅✅   ValidKeys:                                                                                   
                                                                            - default_config_hash                                                                      
   system.site:_core.default_config_hash   Correct   Validatable   ✅✅   NotNull: {  }                                                                                
                                                                          Regex: '/^[a-zA-Z0-9\-_]+$/'                                                                 
                                                                          Length: 43                                                                                   
                                                                          ↣ PrimitiveType: {  }                                                                        
   system.site:admin_compact_mode          Correct   Validatable   ✅✅   ↣ PrimitiveType: {  }                                                                        
   system.site:default_langcode            Correct   Validatable   ✅✅   NotNull: {  }                                                                                
                                                                          Choice:                                                                                      
                                                                            callback: 'Drupal\Core\TypedData\Plugin\DataType\LanguageReference::getAllValidLangcodes'  
                                                                          ↣ PrimitiveType: {  }                                                                        
   system.site:langcode                    Correct   Validatable   ✅✅   NotNull: {  }                                                                                
                                                                          Choice:                                                                                      
                                                                            callback: 'Drupal\Core\TypedData\Plugin\DataType\LanguageReference::getAllValidLangcodes'  
                                                                          ↣ PrimitiveType: {  }                                                                        
   system.site:mail                        Correct   Validatable   ✅✅   Email:                                                                                       
                                                                            message: '%value is not a valid email address.'                                            
                                                                          ↣ PrimitiveType: {  }                                                                        
   system.site:mail_notification           Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here                                                    
   system.site:name                        Correct   Validatable   ✅✅   Regex:                                                                                       
                                                                            pattern: '/([^\PC])/u'                                                                     
                                                                            match: false                                                                               
                                                                            message: 'Labels are not allowed to span multiple lines or contain control characters.'    
                                                                          ↣ PrimitiveType: {  }                                                                        
   system.site:page                        Correct   Validatable   ✅✅   ValidKeys: '<infer>'                                                                         
   system.site:page.403                    Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here                                                    
   system.site:page.404                    Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here                                                    
   system.site:page.front                  Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here                                                    
   system.site:slogan                      Correct   Validatable   ✅✅   Regex:                                                                                       
                                                                            pattern: '/([^\PC])/u'                                                                     
                                                                            match: false                                                                               
                                                                            message: 'Labels are not allowed to span multiple lines or contain control characters.'    
                                                                          ↣ PrimitiveType: {  }                                                                        
   system.site:uuid                        Correct   Validatable   ✅✅   Uuid: {  }                                                                                   
                                                                          NotNull: {  }                                                                                
                                                                          ↣ PrimitiveType: {  }                                                                        
   system.site:weight_select_max           Correct   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=system.site --detail --list-constraints

Proposed resolution

Add validation constraints to:

1. system.site:mail_notification
2. system.site:page.403
3. system.site:page.404
4. system.site:page.front
5. system.site:weight_select_max

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

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

Remaining tasks

User interface changes

None.

API changes

Data model changes

More validation 🚀

Release notes snippet

None.