Empty vertical tabs when mapping schema type overwrites field schema mappings

When using schema-based config forms with vertical tabs, the tabs appear empty due to an array merge operation that overwrites field schema mappings with generic type schema information.

Problem/Motivation

When building form elements for a mapping type, the module merges type schema with field schema using:

$mapping_schema = $type_schema + $field_schema;

This causes the generic type schema's empty `mapping` array to overwrite the field schema's `mapping` array that contains the actual tab content, resulting in empty vertical tabs in the form.

Steps to reproduce

1. Create a config schema using mapping types for vertical tabs structure
2. Define content inside the mappings for each tab
3. Create a form extending SchemaBasedConfigForms' ConfigFormBase
4. Access the form

Current behavior:
- Vertical tabs are rendered but appear empty
- The tab content defined in the field schema mappings is lost

Expected behavior:
- Vertical tabs should display their content as defined in the field schema mappings

Proposed resolution

Invert the array merge operation to preserve field schema mappings:

$mapping_schema = $field_schema + $type_schema;

This ensures that specific field schema definitions take precedence over generic type schema information.

Related code:

// In schema_based_config_forms/src/Form/ConfigFormBase.php, line 185:
$mapping_schema = $type_schema + $field_schema;  // Current
$mapping_schema = $field_schema + $type_schema;  // Proposed