fix: Editor preloading survives blocked asset downloads

[dcalhoun]

What?

Makes the editor resilient to individual asset download failures during preloading, preventing the editor from failing to load when content blockers (like AdGuard) block analytics scripts.

Why?

Fix CMM-1179.

When AdGuard blocks stats.wp.com/w.js, the EditorAssetLibrary.buildBundle() method was failing entirely because it used withThrowingTaskGroup - any single asset failure would cause the entire bundle download to fail, resulting in an "Editor Error" screen with "bad URL" message.

How?

Production code (EditorAssetLibrary.swift)

• Changed buildBundle() from withThrowingTaskGroup to withTaskGroup
• Individual asset download failures are now caught, logged as warnings, and the download continues
• Updated fetchAsset() to return Void instead of URL (return value was unused)

Test code (EditorAssetLibraryTests.swift)

• Simplified EditorAssetLibraryMockHTTPClient by:
  • Making urlResponseHandler throwable: ((URL) throws -> Data)
  • Removing shouldThrowError and downloadResponse properties
  • Using the same handler for both perform() and download() methods
• Added test for graceful handling of blocked assets

Testing Instructions

1. Use a tool to block stats.wp.com in some manner—e.g., AdGuard, Proxyman.
2. Open a post in an app using GutenbergKit
3. Verify the editor loads successfully (previously showed "Editor Error")
4. Check logs for warning about failed asset download

Automated Testing

• Run make test-swift-package - all tests pass including new test buildBundle continues when individual asset downloads fail

[dcalhoun]

Removed in favor of composing the urlResponseHandler for unique scenarios—e.g., URL-specific data and errors.

[jkmassel]

Discussed in Slack – this seems ok – when combined with #301 it'll make it so that we fetch any failed assets at runtime.