feat: content gating navigation API

[sundarthapa2u]

Dual call for gating content logic

• One with None (to fetch all blocks).
• One with request.user (to fetch only visible blocks).
  Compared both trees to identify which units/sections are hidden for the current user.

Previous API example

{
    "blocks": {
        "block-v1:edx+ORA101+2025_09+type@course+block@course": {
            "children": [
                "block-v1:edx+ORA101+2025_09+type@chapter+block@1530cc5e2c1e471fa07c7e9abf9ed5f9"
            ],
            "complete": false,
            "description": null,
            "display_name": "test  course",
            "due": null,
            "effort_activities": null,
            "effort_time": null,
            "icon": null,
            "id": "block-v1:edx+ORA101+2025_09+type@course+block@course",
            "lms_web_url": null,
            "resume_block": false,
            "type": "course",
            "has_scheduled_content": null,
            "hide_from_toc": false,
            "completion_stat": {
                "completion": 0,
                "completable_children": 1
            }
        }
}

After API example

{
    "blocks": {
        "block-v1:edx+ORA101+2025_09+type@course+block@course": {
            "children": [
                "block-v1:edx+ORA101+2025_09+type@chapter+block@1530cc5e2c1e471fa07c7e9abf9ed5f9"
            ],
            "complete": false,
            "description": null,
            "display_name": "test  course",
            "due": null,
            "effort_activities": null,
            "effort_time": null,
            "icon": null,
            "id": "block-v1:edx+ORA101+2025_09+type@course+block@course",
            "lms_web_url": null,
            "resume_block": false,
            "type": "course",
            "has_scheduled_content": null,
            "hide_from_toc": false,
            "ïs_gated": false
            "completion_stat": {
                "completion": 0,
                "completable_children": 1
            }
        }
}

NOTE: is_gated flag

[nsprenkle]

Nit: I would rethink the name is_gated as it doesn't have as much intrinsic meaning / it differs from the other teminology we use across the platform. Perhaps something like is_restricted or access_restricted which is more explicit.

[nsprenkle]

This can be made significantly more efficient. This PR changes from traversing the entire course tree twice to traversing four times times. I'm going to challenge you to think about modifying the get or filter steps to preserve the metadata you want.

[nsprenkle]

Another comment: this should (probably) be feature-flagged or have some other way of selectively including / excluding these fields for use-cases that don't want them.