feat: content-create and content-update with schema validation

All-or-nothing writes: validation (fields, terms, featured image, parent,
template) before any write; failed extras delete the new draft (create) or
restore revision, old columns, fields, terms and featured image (update).
content-update requires rev; Rev::ofPost covers content, title, excerpt,
status and post_modified_gmt. MetaFieldSource::write() now throws on failure.

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

src/Integrations/WordPress/Schema/MetaFieldSource.php

@@ -4,8 +4,11 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Integrations\WordPress\Schema;
 
+use RuntimeException;
+
 /**
- * Shared read/write via the post meta API.
+ * Shared read/write via the post meta API. write() throws when WordPress did not
+ * store a value, so callers can roll back (all or nothing, spec §8.2).
  */
 abstract class MetaFieldSource implements FieldSource
 {
@@ -19,20 +22,50 @@ abstract class MetaFieldSource implements FieldSource
         foreach ($values as $key => $value) {
             $key = (string) $key;
             if ($value === null) {
-                delete_post_meta($postId, $key);
+                $this->delete($postId, $key);
                 continue;
             }
 
             $field = $definitions[$key] ?? null;
             if ($field !== null && $field->multiple && is_array($value) && array_is_list($value)) {
-                delete_post_meta($postId, $key);
+                $this->delete($postId, $key);
                 foreach ($value as $item) {
-                    add_post_meta($postId, $key, wp_slash($item));
+                    if (add_post_meta($postId, $key, wp_slash($item)) === false) {
+                        throw new RuntimeException(sprintf('Meta "%s" of post %d could not be added.', $key, $postId));
+                    }
                 }
                 continue;
             }
 
-            update_post_meta($postId, $key, wp_slash($value));
+            // update_post_meta() also returns false when the value is unchanged – only a
+            // stored value that still differs is a failure.
+            if (update_post_meta($postId, $key, wp_slash($value)) === false && ! $this->holds($postId, $key, $value)) {
+                throw new RuntimeException(sprintf('Meta "%s" of post %d could not be saved.', $key, $postId));
+            }
         }
     }
+
+    private function delete(int $postId, string $key): void
+    {
+        // delete_post_meta() returns false as well when there was nothing to delete.
+        if (delete_post_meta($postId, $key) === false && metadata_exists('post', $postId, $key)) {
+            throw new RuntimeException(sprintf('Meta "%s" of post %d could not be deleted.', $key, $postId));
+        }
+    }
+
+    private function holds(int $postId, string $key, mixed $value): bool
+    {
+        $stored = get_post_meta($postId, $key, false);
+        if (! is_array($stored) || count($stored) !== 1) {
+            return false;
+        }
+
+        $current = $stored[0];
+        if (is_scalar($current) && is_scalar($value)) {
+            // Meta is stored as text: 3 and "3", true and "1" are the same stored value.
+            return (string) $current === (string) $value;
+        }
+
+        return $current == $value; // phpcs:ignore Universal.Operators.StrictComparisons.LooseEqual
+    }
 }