Add global_id to users JSON response and corresponding tests

[albertoleoncio]

What this PR does

• Adds user global_id to the course users JSON payload at /courses/:slug/users.json, aligning JSON output with course_students_csv.
• This change is needed because global_id had already been added to the CSV in Issue #5195 Added global_id column next to username column to CSV export of editor data #5232 (requested by me), but a later change in 3f347ae now requires login before accessing that CSV.
• I've updated my downstream integration to consume JSON instead of CSV. This PR restores parity by exposing global_id in the JSON response as well.
• Adds request-spec coverage to ensure global_id remains present in the users JSON payload.

AI usage

I used GitHub Copilot (GPT-5.3-Codex) to:

• Locate where /courses/:slug/users.json is serialized.
• Add global_id to the users JSON payload.
• Draft and validate a focused request spec for this behavior.

I reviewed the changes and test result manually.

Screenshots

Before:

• Not applicable (no UI change).

After:

• Not applicable (no UI change).