feat: serve PyPI and Simple JSON from the content app

[TenSt]

Add PythonDistribution.content_handler_json so the content app returns the same PyPI JSON and PEP 691 Simple JSON as the /pypi/ API when Accept prefers JSON. Package artifacts stay binary, and Simple JSON uses the official media type so pip can consume it.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough

Note: this PR is dependent on pulp/pulpcore#7996. Making it draft until that one is merged.

[gerrod3]

Why is this needed? Doesn't seem necessary.

[dkliban]

@gerrod3 This is not what i expected this change to bring. The goal is to provide a JSON representation of content present in the repository version associated with the distribution. this is to be used with a UI that shows what packages are in the repository.

[TenSt]

Why is this needed? Doesn't seem necessary.

We had a discussion with @dkliban about it and I will close this one if favor of the future one.