fix: Include metadata for blocked sequence units to enable proper navigation

[efortish]

Backport from #36485

Description

The original issue was that when a sequence was locked due to prerequisites, the API returned an empty items array ([]). This prevented the frontend from knowing what units were inside the locked sequence, meaning it couldn't construct the URLs correctly for navigation so the next/previous buttons stop working.

This modification ensured that even when the sequence is locked, basic metadata for each unit is included. This allows the frontend to:

• Know how many units exist and their IDs

• Correctly construct URLs for "Previous" and "Next" buttons

• Still enforce access restrictions (thanks to the is_gated: True flag)

With this change, the Mfe can continue functioning normally without requiring modifications, as it now consistently receives the necessary information, regardless of whether the sequence is locked or not.

This fix can be backported.

Test cases and test instructions

2025-04-04.12-55-52.mp4

Achievements

• Users can now navigate between sequences correctly, even when some are blocked by prerequisites.
• The prerequisite system’s security and integrity are maintained—the actual content remains blocked.
• No frontend modifications were needed, as the solution is compatible with the existing implementation.
• Significant improvement in user experience, preventing students from getting "stuck" in blocked sequences.

Related issues

#36826
openedx/frontend-app-learning#1546

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[efortish]

@mariajgrimaldi FYI!