Surface an error when the worker script fails to load (#8018)

[clement-igonet]

When the worker script fails to load (wrong setWorkerUrl(), missing self-hosted bundle, blocked import), the map now says so instead of hanging silently: the map fires an error event with an actionable message, and the console shows the same with the worker URL. Only an error arriving before the worker's first message is treated as a load failure, so runtime errors of a healthy worker are not affected.

On the issue's repro, instead of nothing:

MapLibre: The map's worker script failed to load. Sources that depend on the worker (vector tiles, GeoJSON) cannot be processed and the map may never fire "load". When bundling or self-hosting, point setWorkerUrl() at a reachable copy of the worker bundle; see the v5-to-v6 migration guide. Worker URL: ./does-not-exist-worker.mjs

Also fixes the silent hang analyzed in #8074.

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues: Map hangs silently when the worker script fails to load: no error event, no console message, 'load' never fires #8018, 6.0 regression: map never fires load/idle on headless software WebGL (SwiftShader/llvmpipe/lavapipe); 5.24 works #8074
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.
• Confirm you have read our AI policy here.

Assisted-By: Claude (claude-fable-5)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.37%. Comparing base (71480b6) to head (8f7294e).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8089      +/-   ##
==========================================
+ Coverage   93.33%   93.37%   +0.04%     
==========================================
  Files         291      291              
  Lines       24603    24631      +28     
  Branches     6494     6497       +3     
==========================================
+ Hits        22962    22998      +36     
+ Misses       1641     1633       -8     

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[HarelM]

This is just rude now.
There's an open issue and an open PR linked to that issue.
Please stop this AI madness.

[clement-igonet]

This is just rude now. There's an open issue and an open PR linked to that issue. Please stop this AI madness.

#8024 was already open for this and I should have checked before opening anything. Closing was correct.