Add validation constraints to system.theme

Problem/Motivation

system.theme has 2 property path that are not yet validatable:

./vendor/bin/drush config:inspect --filter-keys=system.theme --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.theme                              Correct   60%           ✅❓   ValidKeys: '<infer>'                        
                                                                           LangcodeRequiredIfTranslatableValues: null  
   system.theme:                            Correct   Validatable   ✅✅   ValidKeys: '<infer>'                        
                                                                           LangcodeRequiredIfTranslatableValues: null  
   system.theme:_core                       Correct   Validatable   ✅✅   ValidKeys:                                  
                                                                             - default_config_hash                     
   system.theme:_core.default_config_hash   Correct   Validatable   ✅✅   NotNull: {  }                               
                                                                           Regex: '/^[a-zA-Z0-9\-_]+$/'                
                                                                           Length: 43                                  
                                                                           ↣ PrimitiveType: {  }                       
   system.theme:admin                       Correct   NOT           ✅❓   ⚠️  @todo Add validation constraints here   
   system.theme:default                     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.theme --detail --list-constraints

Proposed resolution

Add validation constraints to:

1. system.theme:admin
2. system.theme:default

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.