[extractor/substack] Return canonical URLs

[handlerug]

Description of your pull request and other information

The URL passed to _real_extract is of format "{username}.substack.com", but for blogs with custom domains the canonical URL would use the custom domain. Because of the wrong URL, the cookies in the resulting info dict come from the wrong domain, which breaks subscriber content extraction.

This isn't really testable because for whatever reason yt-dlp itself doesn't have any trouble downloading such content; however, yt-dlp -j consumers are broken without this change because of how the cookies field is populated.

Template

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check all of the following options that apply:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)

Copilot Summary

🤖 Generated by Copilot at fa51465

Summary

🎥🛠️🔗

Enhanced the Substack extractor to support custom domains. Used canonical_url instead of username for video extraction and added more metadata fields.

Substack extractor
canonical_url for videos
Autumn leaves falling

Walkthrough

• Modify _extract_video_formats function to use url instead of username and urllib.parse.urljoin to construct video URL (link)
• Modify _real_extract function to get domain value from webpage_info and use it to set canonical_url with custom domain if applicable (link)
• Pass canonical_url instead of username to _extract_video_formats function in _real_extract function (link)
• Add webpage_url and original_url fields to metadata dictionary in _real_extract function, using canonical_url as the value (link)