Fix n-tv.de extractor, website format has changed

[1100101]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

As the website format has been changed for videos on n-tv.de, the extractor was broken and could not download videos anymore.

Fixes #3179

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 1dbb4bb

Summary

🛠️🆕🧪

Fix and improve ntvde extractor. Use traverse_obj to handle JSON data and add support for new formats. Update test case with new video.

Oh we're the crew of the ntvde extractor
We fix the bugs and we support new factors
We parse the JSON with traverse_obj
And we update the test case with a heave and a ho

Walkthrough

• Rewrite video extraction and format selection logic for NTV.de (link)
• Import traverse_obj function from utils module to access nested values in JSON data (link)
• Update test case with new video file and its md5 and duration values (link, link)
• Change thumbnail key from html5VideoPoster to poster in yt_dlp/extractor/ntvde.py (link)