Add missing parameter type hints to FieldUiTestTrait

Problem/Motivation

The FieldUiTestTrait contains several methods where parameter type hints are missing, despite the codebase increasingly adopting strict typing.

For example:

public function fieldUIAddNewField($bundle_path, $field_name, $label = NULL, $field_type = 'test_field', array $storage_edit = [], array $field_edit = [], bool $save_settings = TRUE): void

Some parameters such as $bundle_path, $field_name, and $label can be explicitly typed (e.g., ?string, string) based on their usage.

Steps to reproduce

1. Inspect core/modules/field_ui/tests/src/Traits/FieldUiTestTrait.php

2. Review method signatures such as:

* fieldUIAddNewField()
* fieldUIAddExistingField()
* fieldUIDeleteField()
* getFieldFromGroup()

3. Observe that several parameters lack type hints even though their expected types are clear from usage.

Proposed resolution

Add appropriate parameter type hints where applicable, for example:

public function fieldUIAddNewField(
  ?string $bundle_path,
  string $field_name,
  ?string $label = NULL,
  string $field_type = 'test_field',
  array $storage_edit = [],
  array $field_edit = [],
  bool $save_settings = TRUE
): void

Remaining tasks

None.

User interface changes

None.

Introduced terminology

None.

API changes

None.