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fix: dispose Three.js renderer before Babylon.js screenshot to fix black wearable thumbnail (#3414)

* fix: dispose Three.js renderer before Babylon.js screenshot to fix black thumbnail

When a user selects a wearable category, updateThumbnailByCategory calls
getModelData with EngineType.BABYLON. This path first loads the model with
Three.js (creating a WebGL context) and then calls getScreenshot which creates
a second Babylon.js WebGL context — both active simultaneously.

Browsers limit active WebGL contexts to 8-16; having two new contexts alongside
the existing WearablePreview renderer can exhaust the limit, causing Babylon.js
to produce a blank/black screenshot that replaces the valid initial thumbnail.

Fix: dispose the Three.js renderer before calling getScreenshot so only one
WebGL context is active at a time. Also adds renderer cleanup to the catch block
to prevent context leaks when errors occur before the normal disposal point.

Closes https://github.com/decentraland/builder/issues/3413

* fix: dispose Babylon.js engine and scene after screenshot to prevent WebGL context leak

The getScreenshot function was never disposing the Babylon.js Engine or Scene
after taking a screenshot. Each call leaked a WebGL context, and after a few
uploads the browser's context limit (~8-16) was hit, causing subsequent
renders to silently produce empty thumbnails with no error.

Changes:
- Wrap getScreenshot body in try/finally to ensure Engine disposal
- Call scene.dispose() and engine.dispose() after screenshot is captured
- Add webglcontextlost event listener to detect context exhaustion and
  throw an explicit error instead of silently returning an empty image
- Guard canvas.removeChild with parentNode check for safety

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

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