fix: modify pull-through metadata requests to point at Pulp

[am9zZWY]

Assisted By: Claude Sonnet 5

closes #412

📜 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

[dkliban]

Nice fix — the approach is solid and the refactoring is clean. A few suggestions for additional test coverage:

1. Scoped package pull-through — Run the same test_pull_through_install flow with a scoped package (@scope/package). This would exercise URL encoding in _packument_from_remote and the name/-/filename construction for names containing slashes.

2. Local packages path (regression guard) — A test where content is already synced into the repository, then a packument request is made. The refactoring moved this code into _packument_from_local_packages; a test should verify it still returns correct tarball URLs and content_type: application/json.

3. Pull-through with repository + remote (mixed) — A distribution with both a repository containing some cached packages and a remote. First request (no local content) should hit the remote rewrite path; after the tarball is pulled through, a second packument request should come from local packages. Verifies the handoff between the two codepaths.

4. Remote fetch failure — Verify that an unreachable remote returns None gracefully rather than raising.

Items 1 and 2 are the highest value — scoped packages are the most likely real-world breakage, and the local path refactoring needs regression coverage.

[dkliban]

Let's add more tests as described in the comment.