Load og:image preview even if Content-Length header is missing

[der-eismann]

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Description

Fixes #6645. As mentioned in the issue already, a lot of major CDNs (e.g. Amazon's CloudFront) omit the Content-Length header in some cases like when compression is enabled. This means that a lot of preview images cannot be loaded on desktop, but load without any issues on the mobile apps, which is confusing for users.

I changed the behavior to not fail when the header is missing. To still keep it safe I moved the Content-Type check in front of it and added a second one checking the actual size of the returned body before loading the image.

Tested with product pages of the shop I work with, previously the image wouldn't load at all, now it's displayed nicely in the chat. I also added a test to see if the second check was working as expected to still bail on huge images. I tested it on Fedora Linux 40 but no other devices.

(Hidden URLs since they are from an internal staging environment)