[PR #1372/2653a469 backport][3.36] Prefer simple JSON when possible

[patchback]

This is a backport of PR #1372 as merged into main (2653a46).

A more simple fix vs adding proper q-selection negotiation, all the modern tools want to use JSON so it should be default when possible.

fixes: #1370
Generated-by: gpt-5.6-luna

📜 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

Summary by CodeRabbit

• Bug Fixes

  • PyPI Simple API requests now prefer JSON when clients advertise both JSON and HTML formats.
  • HTML remains the default when JSON is not explicitly requested.
  • JSON package detail responses now include upload times for every file.
  • Cached responses for JSON and HTML content types are kept separate, preventing one format from affecting the other.

• Tests

  • Expanded coverage for content negotiation, response fields, and format-specific caching.