Stop shell instructions video reloading on every ActionCable data event

[VasylMarchuk]

Closes #3318

Brief

This fixes the video on the Print a prompt Shell course stage reloading every time ActionCable receives some data.

Details

Fixed by moving the video embed HTML code from the component getter into the template.

After

Before

2025-12-06.12.42.18.mov

Checklist

• I've thoroughly self-reviewed my changes
• I've added tests for my changes, unless they affect admin-only areas (or are otherwise not worth testing)
• I've verified any visual changes using Percy (add a commit with [percy] in the message to trigger)

[github-actions]

Test Results

  1 files  ±0    1 suites  ±0   9m 57s ⏱️ - 5m 12s
681 tests ±0  630 ✅ +2  51 💤 ±0  0 ❌ ±0 
681 runs  ±0  630 ✅ +4  51 💤 ±0  0 ❌  - 2 

Results for commit f9afcca. ± Comparison against base commit 1dac855.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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[rohitpaulk]

@VasylMarchuk how does this work? How come this approach triggers a re-render but the other doesn't? 🤔 I thought if property values don't change after one runloop ember doesn't trigger re-renders?

[rohitpaulk]

This is a pretty unintuitive fix, so want to see if we can either fix an underlying problem so that it's not needed, or come up with a rule of thumb around when to use template vs. properties.

Additionally, I don't think this change will work forever - today the embed html value is hardcoded in the frontend, but in the future it'll come from a property on the CourseStage model. We already have something like this for the CourseStageScreencast model.