Validate attachment child MIME metadata shape

[adamziel]

What it does

Reports present-but-empty or non-string generated-size and backup-size mime-type metadata as plugin-wp-attachment-metadata-invalid-shape in the built-in WordPress upload validator.

Rationale

Malformed mime-type fields are metadata shape problems, not extension drift. Keeping them in the invalid-shape bucket avoids presenting bad metadata as a normal MIME mismatch repair.

Implementation

cow_merge_wordpress_attachment_upload_issues() now validates generated and backup mime-type values before comparing them to the file extension. Invalid values record the affected metadata field, role, value type, attachment file, child file, and upload path evidence.

The media validator fixture adds generated-size and backup-size attachments with empty child MIME metadata and asserts that the built-in validator records invalid-shape conflicts while the built-in MIME-drift count stays unchanged.

Testing instructions

git diff --check 'origin/trunk'...HEAD && git diff --check && git diff --cached --check
php -l scripts/cow/merge.php
php -l tests/cow/media_validator.php
php tests/cow/media_validator.php
php tests/cow/merge_smoke.php
php tests/cow/schema_review.php