Improve handling of large video file attachments

[ianrahman]

First time contributor checklist

• I have read the README and CONTRIBUTING documents
• I have signed the Contributor Licence Agreement

Contributor checklist

• My commits are rebased on the latest main branch
• My commits are in nice logical chunks
• My contribution is fully baked and is ready to be merged as is
• I have tested my contribution on these devices:

• iPhone 16 Pro Max, iOS 26.1 b4

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Addresses #4370

Description

• retry AVAsset exports across presets and map AVError.maximumFileSizeReached to .fileSizeTooLarge
• enforce preset dimension limits based on bitrate estimates and annotate the heuristics we rely on
• add shared helpers for export error detection and size/quality guardrails
• made media file size limits consistent and removed hardcoded calculations for static values

Testing

• Attached a large (> 100 MB) video file to a message and successfully sent to a non-dev device. Received video size was < 100 MB and video quality was significantly better than same video attached and sent with current production implementation.

[sashaweiss-signal]

Hi, thanks for your interest in contributing. This is something we're actively working on improving internally, and these changes will likely be superceded in the near future by that work. The changes here are also substantial, so regardless we'll have to apply a careful review, and unfortunately I can't guarantee a timeline for that.

For the time being I'm going to mark this as Acknowledged and leave the PR open – we'll keep it in mind as we continue our internal improvements, and we'll reply here when we have an update.

Thanks again for your interest, and as always for being a Signal user.