Description

BillboardTexture.loadImage reads atlas.rectangles[index].width after
awaiting TextureAtlas.addImage. In some interleavings — most reliably
when the atlas resizes during the frame the image was queued — that slot
can still be undefined, and the access escapes as an unhandled rejection:

Uncaught (in promise) TypeError: Cannot read properties of undefined
(reading 'width')
    at BillboardTexture.loadImage

This shows up in apps that add/remove billboards rapidly (we hit it
through ol-cesium's FeatureConverter.csAddBillboard on a timer-driven
re-render loop).

This change guards the rectangle access and, when it's missing, marks the
texture as FAILED — matching the existing pattern in the same method
for !defined(index) || index === -1.

Issue number and link

#13486

Testing plan

Added a regression spec covering the case where TextureAtlas.addImage
resolves with an index whose rectangle is undefined. The spec asserts the
texture transitions to BillboardLoadState.FAILED and loadImage does
not reject.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

AI acknowledgment

• I used AI to generate content in this PR
• If yes, I have reviewed the AI-generated content before submitting

If yes, I used the following Tools(s) and/or Service(s):

• Claude

If yes, I used the following Model(s):

• Opus 4.7