[extractor/substack] Fix embed URL extraction

[handlerug]

Description of your pull request and other information

The format of the JSON payload being extracted has changed to a JSON.parse("...")-style format. This PR fixes the extraction process so that it ignores the added slashes.

There's a theoretical possibility of the extracted string being too short if it contains an escape sequence (which would contain a slash thus not matching the [^\"]+ part). In practice, a valid DNS name is unlikely to get escaped by a sensible JSON encoder. I have searched for uses of _search_json and _parse_json and found none, so I opted for a fix to the regular expression.

Regarding tests, there are two in generic.py which should have actually been failing. If I try them manually using yt-dlp -j, the result seems to match, but I don't know how to run specific download tests in a generic way.

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Copilot Summary

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Summary

🐛🔧🧪

Fixed substack extractor to handle backslashes in subdomain names. Modified regex pattern in yt_dlp/extractor/substack.py.

Substack posts vary
Escape backslashes in regex
Winter bug is fixed

Walkthrough

• Escape backslashes in subdomain pattern (link) to fix extraction of some substack posts in yt_dlp/extractor/substack.py