Tighten validation of `experience_builder.generated_field_explicit_input_ux: prop_field_definitions.[%key].expression`

Overview

In HEAD:

Proposed resolution

1. Make this test coverage (copy/paste into ComponentValidationTest) pass:
   

     /**
      * @testWith ["ℹ︎non_existing_field_type␟value", "Invalid expression: the \"non_existing_field_type\" field type does not exist."]
      *           ["ℹ︎string␟non_existing_field_property", "Invalid expression: the \"string field\" type does not have a field property \"non_existing_field_property\"."]
      *           ["nonsense", "Invalid structured data expression."]
      *           ["ℹ︎␜entity:node␝title␞␟value", "A \"FieldPropExpression\" was provided, but expected one of \"FieldTypePropExpression\", \"FieldTypeObjectPropsExpression" or \"ReferenceFieldTypePropExpression\"."]
      *
      * @see `type: experience_builder.generated_field_explicit_input_ux`
      * @covers \Drupal\experience_builder\Plugin\ExperienceBuilder\ComponentSource\GeneratedFieldExplicitInputUxComponentSourceBase
      */
     public function testInvalidPropFieldDefinitionExpression(string $expression, string $expected_message): void {
       $settings = $this->entity->getSettings();
       $settings['prop_field_definitions']['text']['expression'] = $expression;
       $this->entity->setSettings($settings);
       $this->assertValidationErrors(['settings.prop_field_definitions.text.default_value' => $expected_message]);
     }
   

2. Add this to config schema:
   

   diff --git a/config/schema/experience_builder.schema.yml b/config/schema/experience_builder.schema.yml
   index 5f8d95ecb..9e8fd23e8 100644
   --- a/config/schema/experience_builder.schema.yml
   +++ b/config/schema/experience_builder.schema.yml
   @@ -394,13 +394,15 @@ experience_builder.generated_field_explicit_input_ux:
                  type: field.value.[%parent.%parent.field_type]
                # @todo Add validation constraint because this is only *conditionally* nullable: if the SDC prop is optional
                nullable: true
   -          # The default value must be retrievable
   +          # The default value must be retrievable.
              expression:
                type: string
   -            label: ''
   -            constraints: { }
   -            # @todo Add `StructuredDataPropExpression` validation constraint that fails if StructuredDataPropExpression::fromString() fails
   -            # StructuredDataPropExpression: [field_type]
   +            label: 'The expression'
   +            constraints:
   +              ValidStructuredDataPropExpression:
   +                - FieldTypePropExpression
   +                - FieldTypeObjectPropsExpression
   +                - ReferenceFieldTypePropExpression
    
    experience_builder.component_source_settings.js:
      constraints:
   

3. Create the ValidStructuredDataPropExpression validation constraint to make the above test coverage pass :)

User interface changes

None.