Add the missing configuration schema

Problem/Motivation

The module ships no config/schema directory, so none of the configuration it writes has a schema. On a site with the configuration schema checker on, two things are reported.

Saving /admin/media-pdf-thumbnail/settings/global:

No schema for media_pdf_thumbnail.settings

Saving an entity view display whose image field uses the Media PDF Thumbnail Image formatter:

Schema errors for core.entity_view_display.media.document.media_library with the
following errors: ...:content.thumbnail.settings.pdf_file_field missing schema,
...settings.pdf_page missing schema, ...  [60 keys on a standard profile]

Every key is reported, image_style, image_link and image_loading included, because with no field.formatter.settings.media_pdf_thumbnail_image_field_formatter entry nothing resolves - not even the three keys the formatter only inherits from the image formatter it extends.

Steps to reproduce

Drupal 11.4.6, media_pdf_thumbnail 7.0.0, standard profile.

1. Install the module.
2. Save the global settings form. -> first message.
3. Set thumbnail on a media view display to the Media PDF Thumbnail Image formatter and save. -> second message.

drush config:inspect --only-error shows the same thing: media_pdf_thumbnail.settings No schema, and 60 missing schema keys under the display.

Proposed resolution

config/schema/media_pdf_thumbnail.schema.yml covering the two config objects. media_pdf_thumbnail.settings is a plain two-key config_object. The formatter entry is type: field.formatter.settings.image plus its own keys, the way core's field.formatter.settings.media_thumbnail extends its parent.

The per-bundle keys cannot go in the YAML: getDefaultSettings() seeds ten settings per media bundle, keyed <bundleId>_page, <bundleId>_field and so on, and config schema has no wildcard for keys inside a mapping. hook_config_schema_info_alter() adds them at runtime, the way ckeditor5_config_schema_info_alter() does for its own plugin-named keys.

Because the typed configuration definitions are cached with no cache tag, hook_ENTITY_TYPE_insert() / hook_ENTITY_TYPE_delete() on media_type clear them, so a bundle added later is covered in the same request - a configuration import that creates a media type and a display together, for instance. Without that, the schema only lists the bundles that existed when the definitions were last cached.

Remaining tasks

Six settings hold a different PHP type depending on how the value got there, and SchemaCheckTrait::checkValue() compares types strictly, so naming any one type makes the others an error. They are typed ignore:

A checked checkbox writes the string '1' and one that is not checked the integer 0, while the defaults seed an empty string; pdf_page is seeded as an integer and comes back from its number element as a string. <bundle>_page is consistent - always a string - and is typed properly.

The clean fix is to normalize what the plugin and the form write and add a hook_update_N rewriting the displays and views already saved, after which those keys can be typed properly. That is a behaviour change and is left for a follow-up; ignore has precedent in core outside tests, in layout_builder.schema.yml and field.schema.yml.

User interface changes

None.

API changes

None.

Data model changes

Adds a configuration schema. No stored configuration changes.