refactor: DiviWriteAbility base and shared DiviErrors (H4)

The five Divi write tools extend DiviWriteAbility (final permission and
execute, change() per tool, shared output schema). DiviErrors holds the
unknown-node, position and compose-error mapping; DiviContentComposer uses
the same mapping (ledger T9). ToolNamingTest skips abstract classes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Divi/Pages/DiviErrors.php

@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Pages;
+
+use LIT\MCP\Abilities\Errors;
+use WP_Error;
+use WP_Post;
+
+/** The client errors the Divi tools share. */
+final class DiviErrors
+{
+    /** A path ID that names no node of $post. */
+    public static function unknownNode(string $id, WP_Post $post): WP_Error
+    {
+        return Errors::notFound(
+            sprintf('Knoten "%s" gibt es in Eintrag %d nicht.', $id, $post->ID),
+            DiviPageLoader::OUTLINE_HINT
+        );
+    }
+
+    /** A parent / before / after / index the tree has no place for (TreeEditor::position()). */
+    public static function position(string $problem): WP_Error
+    {
+        return Errors::invalidInput('Ungültige Position; nichts wurde gespeichert.', ['position' => $problem]);
+    }
+
+    /**
+     * The errors of Composer::compose()/update(): invalid_attr_path when an attribute path is
+     * unknown (the more useful hint), else invalid_input with $message.
+     *
+     * @param array{errors: array<string, string>, invalid_paths: list<string>} $result
+     */
+    public static function composed(array $result, string $message): WP_Error
+    {
+        return $result['invalid_paths'] !== []
+            ? Errors::invalidAttrPath($result['errors'])
+            : Errors::invalidInput($message, $result['errors']);
+    }
+}