fix: restrict media-search to the query, not just editable() after it

I1: query the attachments themselves author-restricted when the user
lacks edit_others_posts (PostGuard::ownPostsOnly, extracted from
ContentQueryAbility so content-query reuses the exact same rule)
instead of relying solely on a post-query editable() filter, which
could leak the existence of other users' attachments via an empty
page plus next_cursor. editable() stays as a backstop.

Also: M1 document that only status=inherit attachments are listed;
M2 tie-break ordering by ID (content-query and media-search) for
stable pagination; M3 no next_cursor once the last candidate was
examined; M4 reject an explicit non-image mime with missing_alt;
M5 additional coverage (posts_per_page, cursor offset pass-through,
invalid cursor, post_status/date/mime args); mime maxLength 100.

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

tests/Unit/Integrations/WordPress/Content/ContentQueryAbilityTest.php

@@ -37,8 +37,7 @@ final class ContentQueryAbilityTest extends TestCase
                 && $args['s'] === 'Anna'
                 && $args['posts_per_page'] === 3
                 && $args['offset'] === 4
-                && $args['orderby'] === 'title'
-                && $args['order'] === 'ASC'
+                && $args['orderby'] === ['title' => 'ASC', 'ID' => 'ASC']
                 && $args['tax_query'] === [
                     'relation' => 'OR',
                     ['taxonomy' => 'category', 'field' => 'term_id', 'terms' => [4]],