add option for users to make posts/replies that are only visible to instructors

[pebble-fish]

##User Story##
Students may want to ask questions about a course to instructors, but need to show instructors parts of their buggy code. For many CS classes, it is not allowed to show any part of your code to other students, even if unintentionally. Without the option to make posts visible only to instructors, they risk an AIV or their question going unanswered.

##Summary of PR Additions##

1. The button for Anonymous posting was modified from a toggle to a visibility dropdown in both composers (for QuickReply and NewTopic).
   a. The dropdown has three options: Post Publicly (default), Post Anonymously, and Post to Instructors.
   b. These changes were applied in QUick Reply and New Topic.
   c. the selection is stored in variable visibilityMode
2. Persists visbility mode on posts/topics: posts store visbilityMode (public | anonymous | instructors), and the previous anonymous variable (to ensure the backend stuff still works from previous edits).
3. Introduced canonical visbility helper logic for mode normalization and read checks
   a. instructor only visibility is restricted to: post/topic author or admins
4. Anonymous behavior is still compatible: existing anonymous masking behavior is still preserved, and internal logic now maps anoynmous mode from visibilityMode
5. Added tests for visibility modes to cover public visibility, anonymous visibility persistence/masking, instructor-only visibility for admin/author/regular users, and instructor-only topic filtering behavior.

##Post##

Recording.2026-02-27.193434.mp4

##Quick Reply##

Recording.2026-02-27.193434.mp4

##Errors Encountered##

1. Schema validation failure in API tests

• Failure: anonymous existed in response but wasn’t declared in PostObject schema.
• Fix: added anonymous to public/openapi/components/schemas/PostObject.yaml.

2. Deleted raw post regression

• Failure: test expected null for deleted raw post read by non-mod; code returned raw content.
• Cause: owner-read path inadvertently allowed in api.posts.getRaw.
• Fix: restored strict behavior so deleted raw is only available to admin/mod.

3. Composer/preview/posting regression risk

• Symptom: posting and preview broke in runtime.
• Cause: compatibility break between new dropdown and legacy anonymous checkbox assumptions.
• Fix: added compatibility fallback + hidden legacy checkbox sync in composer and quick reply scripts/templates.

4. Local test environment noise

• Intermittent warnings unrelated to this feature:
  sendmail-not-found
  unsupported test image format warning
• These are pre-existing environment/tooling artifacts and not introduced by visibility logic.

5. Issue with COveralls being down for several days

##Testing##

• build and run via ./nodebb build and ./nodebb restart
• run full test suite using ``npm test`
• manual checks:
  • open quickreply and post, confirm posting as dropdown appears with default as post publically
  • create one post/topic per node: public, anonymous, instructors

[568cats]

Could you add a description of the feature/changes made as well as a way to test the feature in the body of the PR?

[pebble-fish]

Could you add a description of the feature/changes made as well as a way to test the feature in the body of the PR?

updated, i will add the info on how to test later

[sliang-code]

All code are good and it might be better with some additional comments especially in new file like visibility.js.