Move GD-specific code from Rotate::validateArguments() to Rotate::execute()

This took awhile to review thoroughly, but it is pretty close to ready. Nice work!

1. +++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
   @@ -37,12 +37,37 @@ protected function arguments() {
   +    if (!is_numeric($arguments['degrees'])) {
   

   Presumably it should also be between -360 and 360? This might be followup territory since technically someone rotating 477 degrees might experience a BC break. I tested in the user interface and entering e.g. "-477" in the widget works fine. It is odd/confusing with the "Randomize" option though.

2. +++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
   @@ -37,12 +37,37 @@ protected function arguments() {
   +      throw new \InvalidArgumentException('Invalid degrees (' . $arguments['degrees'] . ') specified for the image \'rotate\' operation. It should be numeric.');
   ...
   +      throw new \InvalidArgumentException('Invalid background color (' . $arguments['background'] . ') specified for the image \'rotate\' operation. It should be a hex string.');
   

   I confirmed that the method is already documented as @throws for this exception on the interface.

   Should we maybe double-quote these strings to avoid needing to escape the single quotes and to avoid the concatenation with the arguments?

3. +++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
   @@ -37,12 +37,37 @@ protected function arguments() {
   +    // Assure background color is a valid hex string.
   

   "Assure" does not make sense in the context. It's a transitive verb that means "to soothe doubts", e.g., "I assured them that I would review the patch carefully." This should be:

   Ensure that the background color is a valid hex string.

4. +++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
   @@ -37,12 +37,37 @@ protected function arguments() {
   +  /**
   +   * {@inheritdoc}
   +   */
   +  protected function execute(array $arguments) {
   +    // PHP installations using non-bundled GD do not have imagerotate.
   +    if (!function_exists('imagerotate')) {
   +      $this->logger->notice('The image %file could not be rotated because the imagerotate() function is not available in this PHP installation.', [
   +        '%file' => $this->getToolkit()->getSource(),
   +      ]);
   +      return FALSE;
   +    }
   
   @@ -80,19 +105,6 @@ protected function validateArguments(array $arguments) {
   -  /**
   -   * {@inheritdoc}
   -   */
   -  protected function execute(array $arguments) {
   -    // PHP installations using non-bundled GD do not have imagerotate.
   -    if (!function_exists('imagerotate')) {
   -      $this->logger->notice('The image %file could not be rotated because the imagerotate() function is not available in this PHP installation.', ['%file' => $this->getToolkit()->getSource()]);
   -      return FALSE;
   -    }
   

   I verified that this is just a diff artifact. These lines are not actually changing. They just appear to be moved because a larger number of lines are moved from one method to another.

5. +++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
   @@ -37,12 +37,37 @@ protected function arguments() {
        // PHP 5.5 GD bug: https://bugs.php.net/bug.php?id=65148: To prevent buggy
        // behavior on negative multiples of 90 degrees we convert any negative
        // angle to a positive one between 0 and 360 degrees.
        $arguments['degrees'] -= floor($arguments['degrees'] / 360) * 360;
   

   Out of scope, but maybe a followup: Does this bug still exist? It's been five years since Drupal supported PHP 5.5.

6. +++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
   @@ -80,19 +105,6 @@ protected function validateArguments(array $arguments) {
   -    return $arguments;
   -  }
   -
   

   At a glance the diff makes it look like this is removing the return value, but it is just a diff artifact due to the lines being moved. I confirmed that execute() never returns $arguments in HEAD, only Booleans.

7. This should probably have a change record, since it will change what happens when for contrib or custom code that extends it.

NW primarily for point 3 and for the change record, though followups (or finding existing related issues) would be helpful too. Thanks!

📌 Remove PHP 5 code for GD Needs work addresses #72.5.