fix(scene): report missing prefab UUID on open

[bofeng-song]

Related issue: https://zentao.sud.center/index.php?m=bug&f=view&bugID=833

Summary

• Include the missing Prefab UUID when opening a scene that references a deleted asset.
• Recognize browser Asset fetch failed (404) errors and existing .json or .bin download failures.
• Return the common 404 Not Found status for missing scene dependencies.

Reproduction

1. Add a Prefab to a scene and save the scene.
2. Delete the Prefab asset.
3. Open the scene through the scene API.

The response now identifies the missing asset UUID instead of returning a 404 without actionable details.

Testing

• Added coverage for UUID extraction from browser asset-fetch errors.
• Added coverage for the scene-open 404 response and missing UUID.
• Verified 3 test suites with 43 passing tests.

[bofeng-song]

Current response:

{
"result": {
"code": 404,
"reason": "The asset db://assets/scene-001.scene cannot be loaded because a dependent asset is missing: d6a5996b-cc11-473b-a730-c1d23c778409"
}
}