Media: use updated file path for attachment metadata

[ArkaPrabhaChowdhury]

Description

When an add_attachment callback changes an attachment's file, media metadata generation currently uses the stale pre-insert path. Use get_attached_file() after insertion so uploads and sideloads generate metadata for the current file.

Includes regression tests for both paths.

Trac: https://core.trac.wordpress.org/ticket/35593

Testing

• PHP lint
• PHPCBF
• PHPCS
• PHPUnit could not run because wp-tests-config.php is missing and Docker is unavailable.

[github-actions]

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