Add validation constraints to system.mail

Problem/Motivation

The System module's mail have 4 property paths that are not yet validatable:

vendor/bin/drush config:inspect --filter-keys=system.mail --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.mail                              Correct   67%           ✅❓   ValidKeys: '<infer>'                                                         
                                                                          LangcodeRequiredIfTranslatableValues: null                                   
   system.mail:                            Correct   Validatable   ✅✅   ValidKeys: '<infer>'                                                         
                                                                          LangcodeRequiredIfTranslatableValues: null                                   
   system.mail:_core                       Correct   Validatable   ✅✅   ValidKeys:                                                                   
                                                                            - default_config_hash                                                      
   system.mail:_core.default_config_hash   Correct   Validatable   ✅✅   NotNull: {  }                                                                
                                                                          Regex: '/^[a-zA-Z0-9\-_]+$/'                                                 
                                                                          Length: 43                                                                   
                                                                          ↣ PrimitiveType: {  }                                                        
   system.mail:interface                   Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here                                    
   system.mail:interface.default           Correct   Validatable   ✅✅   PluginExists:                                                                
                                                                            manager: plugin.manager.mail                                               
                                                                            interface: Drupal\Core\Mail\MailInterface                                  
                                                                          ↣ PrimitiveType: {  }                                                        
   system.mail:mailer_dsn                  Correct   Validatable   ✅✅   ValidKeys: '<infer>'                                                         
   system.mail:mailer_dsn.host             Correct   Validatable   ✅✅   NotBlank:                                                                    
                                                                            message: 'The mailer DSN must contain a host (use "default" by default).'  
                                                                          ↣ PrimitiveType: {  }                                                        
   system.mail:mailer_dsn.options          Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here                                    
   system.mail:mailer_dsn.password         Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here                                    
   system.mail:mailer_dsn.port             Correct   Validatable   ✅✅   Range:                                                                       
                                                                            min: 0                                                                     
                                                                            max: 65535                                                                 
                                                                          ↣ PrimitiveType: {  }                                                        
   system.mail:mailer_dsn.scheme           Correct   Validatable   ✅✅   NotBlank:                                                                    
                                                                            message: 'The mailer DSN must contain a scheme.'                           
                                                                          ↣ PrimitiveType: {  }                                                        
   system.mail:mailer_dsn.user             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.mail --detail --list-constraints

Proposed resolution

1. Add validation constraints.
2. Mark FullyValidatable.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet