Add validation constraints to contact.settings

Problem/Motivation

The Contact module's settings have 3 property paths that are not yet validatable:

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

 --------------------------------------------- ------------- ------------------------------------------ 
  Key                                           Validatable   Validation constraints                    
 --------------------------------------------- ------------- ------------------------------------------ 
  contact.settings                              63%           ValidKeys: '<infer>'                      
   contact.settings:                            Validatable   ValidKeys: '<infer>'                      
   contact.settings:_core                       Validatable   ValidKeys:                                
                                                                - default_config_hash                   
   contact.settings:_core.default_config_hash   Validatable   NotNull: {  }                             
                                                              Regex: '/^[a-zA-Z0-9\-_]+$/'              
                                                              Length: 43                                
                                                              ↣ PrimitiveType: {  }                     
   contact.settings:default_form                NOT           ⚠️  @todo Add validation constraints here  
   contact.settings:flood                       Validatable   ValidKeys: '<infer>'                      
   contact.settings:flood.interval              NOT           ⚠️  @todo Add validation constraints here  
   contact.settings:flood.limit                 NOT           ⚠️  @todo Add validation constraints here  
   contact.settings:user_default_enabled        Validatable   ↣ PrimitiveType: {  }                     
 --------------------------------------------- ------------- ------------------------------------------ 

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=contact.settings --detail --list-constraints

Proposed resolution

1. Add validation constraints.
2. Mark FullyValidatable.

Remaining tasks

Review.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

N/A