Apply missing dimensions to images on the query string version that will be visited by SaaS

[remyperona]

Scope a solution ✅

Engine\Media\ImageDimensions\Subscriber

• Add a new callback method start_buffer() on template_redirect action (priority 3)
• The start_buffer() method will perform checks to validate if the buffering should start. This includes is_admin(), is_preview(), REST_REQUEST, and the presence of the query string wpr_imagedimensions. If everything pass, we can start the buffer with the following ob_start( [ $this, 'specify_image_dimensions' ] );. It will fall back to the existing method callback.
• Add tests

Estimate the effort ✅

Effort [S]

[Miraeld]

Seems okay to me.

[MathieuLamiot]

Discussed here: https://wp-media.slack.com/archives/CUT7FLHF1/p1709822793531749 Bringing back to grooming for now to investigate if the PHP approach will cover all cases for the SaaS or not.