Add embeddings and moderation operation definitions to api_defaults.yml

Added embeddings and moderation operation definitions to api_defaults.yml. The file previously only defined chat, but the provider supports all three operations.

Now getApiDefinition() returns complete API documentation for all supported operations:

drush php-eval "                                                                                                                                            
    \$provider = \Drupal::service('ai.provider')->createInstance('mistral');                                                                                                          
    \$def = \$provider->getApiDefinition();                                                                                                                                           
    print 'Operations defined: ' . implode(', ', array_keys(\$def)) . PHP_EOL;                                                                                                        
    foreach (['chat', 'embeddings', 'moderation'] as \$op) {                                                                                                                          
      \$desc = \$def[\$op]['input']['description'] ?? 'MISSING';                                                                                                                      
      print \"  \$op: \$desc\" . PHP_EOL;                                                                                                                                             
    }                                                                                                                                                                                 
  "