feat: fetch scene files from asset bundle CDN instead of catalyst

[decentraland-bot]

Pull Request Description

What does this PR change?

When a scene has asset bundles available, redirects its core files (main script, scene.json, main.crdt) to the asset bundle CDN instead of the catalyst. Before fetching from CDN, performs parallel HEAD requests to validate all three files exist — if any is missing, falls back entirely to catalyst (all-or-nothing).

This is done by injecting CDN URL overrides directly into SceneHashedContent via a new OverrideContentUrl method.

Scenes marked as fallback could fail to load because files were missing from the catalyst. Routing requests to the CDN — where the asset-bundle-converter uploads them — eliminates this class of failure.

Addresses #7625

Test Instructions

Prerequisites

• Use a build from this branch

Test Steps

1. Go to Genesis Plaza in zone (the scene has already been rebuilt with CDN files available)
2. Verify the scene loads correctly and all content renders as expected
3. Go to Genesis Plaza in org
4. Verify the scene also loads correctly — in this case CDN files are not available, so it should fall back to catalyst seamlessly
5. Navigate to other scenes and verify normal loading behavior is not affected

Additional Testing Notes

• The fallback is all-or-nothing: if any of the three HEAD requests fail, all files come from catalyst
• CDN URL pattern: {assetBundleURL}/{abVersion}/{entityId}/{fileName}
• The main script file name is taken from definition.metadata.main (not hardcoded to index.js)

Quality Checklist

• Changes have been tested locally
• Documentation has been updated (if required)
• Performance impact has been considered
• For SDK features: Test scene is included

Code Review Reference

Please review our Code Review Standards before submitting.

[github-actions]

Windows and Mac build successful in Unity Cloud! You can find a link to the downloadable artifact below.

Name	Link
Commit	b34decb
Logs	https://github.com/decentraland/unity-explorer/actions/runs/23948739739
Download Windows	https://github.com/decentraland/unity-explorer/suites/63227359105/artifacts/6260794842
Download Windows S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/fix/7625-cdn-scene-files/pr-20659-b34decb/Decentraland_windows64.zip
Download Mac	https://github.com/decentraland/unity-explorer/suites/63227359105/artifacts/6260710610
Download Mac S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/fix/7625-cdn-scene-files/pr-20659-b34decb/Decentraland_macos.zip
Built on	2026-04-03T14:40:16Z

[AnsisMalins]

feat: fetch scene files from asset bundle CDN instead of catalyst

PR number: 7833
Windows change build
Windows baseline build
macOS change build
macOS baseline build

Intel Core i3

Change samples: 2680
Baseline samples: 2679

Metric	Change	Baseline	Delta	Improvement
CPU average	33.5 ms	33.5 ms	-0.0 ms	0.1%	⚪
CPU 1% worst	49.3 ms	51.1 ms	-1.8 ms	3.5%	🟢
CPU 0.1% worst	101.7 ms	114.5 ms	-12.8 ms	11.1%	🟢
GPU average	15.7 ms	15.6 ms	0.2 ms	-1.0%	⚪
GPU 1% worst	21.6 ms	20.3 ms	1.3 ms	-6.5%	🔴
GPU 0.1% worst	25.8 ms	22.5 ms	3.4 ms	-15.0%	🔴

[AnsisMalins]

PR #7833, run #23950079654

Builds: Windows change, Windows baseline, macOS change, macOS baseline

Intel Core i3

Metric	Change	Baseline	Delta	Improvement
Samples	2675	2674
CPU average	33.6 ms	33.6 ms	-0.0 ms	0.0%	⚪
CPU 1% worst	54.5 ms	55.7 ms	-1.2 ms	2.2%	⚪
CPU 0.1% worst	102.3 ms	136.0 ms	-33.7 ms	24.8%	🟢
GPU average	16.3 ms	15.3 ms	1.1 ms	-7.0%	🔴
GPU 1% worst	22.7 ms	22.3 ms	0.5 ms	-2.0%	⚪
GPU 0.1% worst	31.1 ms	28.0 ms	3.1 ms	-11.2%	🔴