MDL-76723 core_external: Handle format properties properly

- If a text format property defines the 'optional' attribute as true,
then the $required parameter that
is passed to \core\external\exporter::get_format_structure() should be
VALUE_OPTIONAL.
- If a default value is provided for a format property,
\core\external\exporter::get_format_structure() should pass the default
value to the external_format_value constructor.
- Added validation for external_format_value for the default parameter.
  Debugging will be shown in case an invalid default value is passed.
- Amended unit tests for better coverage.

lib/external/classes/external_format_value.php

@@ -38,12 +38,19 @@ class external_format_value extends external_value {
      * @param int $default Default value.
      */
     public function __construct($textfieldname, $required = VALUE_REQUIRED, $default = null) {
+        // Make sure the default format's value is correct.
+        if ($default !== null && !in_array($default, [FORMAT_MOODLE, FORMAT_HTML, FORMAT_PLAIN, FORMAT_MARKDOWN])) {
+            debugging("Invalid default format for $textfieldname: $default. " .
+                      "It must be either FORMAT_MOODLE, FORMAT_HTML, FORMAT_PLAIN, or FORMAT_MARKDOWN.", DEBUG_DEVELOPER);
+            $default = null;
+        }
+
         if ($default == null && $required == VALUE_DEFAULT) {
             $default = FORMAT_HTML;
         }
 
         $desc = sprintf(
-            "%s format (%s = HTML, %s = MOODLE, %s = PLAIN, or %s = MARKDOWN",
+            "%s format (%s = HTML, %s = MOODLE, %s = PLAIN, or %s = MARKDOWN)",
             $textfieldname,
             FORMAT_HTML,
             FORMAT_MOODLE,