fix: enforce courses.manage_pages_and_resources on ProctoredExamSettingsView

[efortish]

Description

ProctoredExamSettingsView (GET and POST) only checked studio read access. That meant a user who could view a course in Studio could also read or overwrite its proctored exam settings by calling the API directly, bypassing the UI's own staff-only restriction. This adds the courses.manage_pages_and_resources authz check to both methods, with a write-level legacy fallback for courses that don't have authz course authoring enabled, matching the pattern already used for the same permission in cms/djangoapps/contentstore/rest_api/v0/views/tabs.py.

Closes openedx-authz#443

Testing

Added test_authz_user_allowed and test_authz_user_not_allowed (shared by the GET and POST test classes), plus verified the existing 31 tests in the file still pass, including the legacy read/write role checks.

To confirm the new tests actually catch the bug and aren't passing by coincidence, I reverted the fix locally and reran them: test_authz_user_not_allowed failed with assert 200 == 403 (a user denied by authz still got through), and test_authz_user_allowed failed because the authz check was never even called. Restoring the fix brings all 35 tests in the file back to green.

[openedx-webhooks]

Thanks for the pull request, @efortish!

This repository is currently maintained by @openedx/wg-maintenance-openedx-platform-oncall.

Once you've gone through the following steps feel free to tag them in a comment and let them know that your changes are ready for engineering review.

🔘 Get product approval

If you haven't already, check this list to see if your contribution needs to go through the product review process.

• If it does, you'll need to submit a product proposal for your contribution, and have it reviewed by the Product Working Group.
  • This process (including the steps you'll need to take) is documented here.
• If it doesn't, simply proceed with the next step.

🔘 Provide context

To help your reviewers and other members of the community understand the purpose and larger context of your changes, feel free to add as much of the following information to the PR description as you can:

• Dependencies

  This PR must be merged before / after / at the same time as ...

• Blockers

  This PR is waiting for OEP-1234 to be accepted.

• Timeline information

  This PR must be merged by XX date because ...

• Partner information

  This is for a course on edx.org.

• Supporting documentation
• Relevant Open edX discussion forum threads

🔘 Get a green build

If one or more checks are failing, continue working on your changes until this is no longer the case and your build turns green.

Details

---

Where can I find more information?

If you'd like to get more details on all aspects of the review process for open source pull requests (OSPRs), check out the following resources:

• Overview of Review Process for Community Contributions
• Pull Request Status Guide
• Making changes to your pull request

When can I expect my changes to be merged?

Our goal is to get community contributions seen and reviewed as efficiently as possible.

However, the amount of time that it takes to review and merge a PR can vary significantly based on factors such as:

• The size and impact of the changes that it introduces
• The need for product review
• Maintenance status of the parent repository

💡 As a result it may take up to several weeks or months to complete a review and merge your PR.