docs: Complete method signatures in example code

Add method bodies ({ ... }) to all example method signatures to make
the code syntactically complete and easier to understand.

Address CodeRabbit review comment.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: koriym

src-annotation/CacheableResponse.php

@@ -22,9 +22,15 @@
  * class Article extends ResourceObject
  * {
  *     public function onGet(int $id): static
+ *     {
+ *         // ...
+ *     }
  *
  *     #[RefreshCache]
  *     public function onDelete(int $id): static
+ *     {
+ *         // ...
+ *     }
  * }
  * ```
  *

src-annotation/DonutCache.php

@@ -22,6 +22,9 @@
  * {
  *     #[Embed(rel: 'comment', src: 'app://self/comments')]
  *     public function onGet(int $id): static
+ *     {
+ *         // ...
+ *     }
  * }
  * ```
  *

src-annotation/Purge.php

@@ -20,6 +20,9 @@
  * #[Purge(uri: 'app://self/user/profile?user_id={id}')]
  * #[Purge(uri: 'app://self/user/friend?user_id={id}')]
  * public function onDelete(int $id): static
+ * {
+ *     // ...
+ * }
  * ```
  *
  * @see https://bearsunday.github.io/manuals/1.0/en/cache.html#tag-based-cache-invalidation

src-annotation/Refresh.php

@@ -19,6 +19,9 @@
  * ```php
  * #[Refresh(uri: 'app://self/user/profile?user_id={id}')]
  * public function onPut(int $id, string $name): static
+ * {
+ *     // ...
+ * }
  * ```
  *
  * @see https://bearsunday.github.io/manuals/1.0/en/cache.html#event-driven-content