[Selection] keep image selection when 'selectionchange' range is collapsed

[liangdrime]

Summary

This change is to prevent SelectionPlugin from converting an image selection into a range selection when the browser reports a collapsed range during "selectionchange".

Issue

Fix selection with ctrl+a #2556

// If am image selection changed to a wider range due a keyboard event, we should update the selection

The logic for converting image selection to a range was first introduced in #2566. According to its description, this change was intended to correct the selection to a range when it expands to cover a wider area, such as when triggered by keyboard shortcuts like Ctrl+A.

However, on Mac, right-clicking an image while it is in editing mode converts the collapsed image selection to a range selection, causing the subsequent 'Crop' action from the context menu to fail.

onSelectionChange currently converts image selection to range whenever isSingleImageInSelection(range) returns null.
For collapsed ranges, that helper correctly returns null, but this was treated as a wider-range transition and caused an unintended downgrade from image selection to collapsed range selection.

Fix

The intended conversion is for wider range transitions only. A collapsed range is an empty caret state, not a wider selection, so converting image selection in that case is not semantically correct and causes product regressions.

So, this change updated SelectionPlugin.onSelectionChange and image to range conversion only happens when the current range is not collapsed.

[liangdrime]

https://github.com/microsoft/roosterjs/actions/runs/29032470134/job/86168595237?pr=3396

Chrome 150.0.0.0 (Linux x86_64) retrieveHtmlInfo Has global CSS rule FAILED
	Error: Expected $.globalCssRules[2].text = 'border-width: medium; border-style: none; border-color: currentcolor; border-image: none;' to equal 'border-width: medium; border-style: none; border-color: currentcolor; border-image: initial;'.
	    at <Jasmine>
	    at runTest (webpack://roosterjs/packages/roosterjs-content-model-core/test/command/paste/retrieveHtmlInfoTest.ts:19:24 <- karma.test.all.2302904290.js:71236:24)
	    at UserContext.<anonymous> (webpack://roosterjs/packages/roosterjs-content-model-core/test/command/paste/retrieveHtmlInfoTest.ts:203:9 <- karma.test.all.2302904290.js:71355:9)
	    at <Jasmine>

The test-on-chrome failed in retrieveHtmlInfoTest.ts. Because the newer Chrome (the CI runner is on 150) expands the border: none shorthand's border-image longhand to none instead of initial.

The fix is simple: updates border-image: initial to border-image: none. However, if this test does not block the change, it should be addressed in a separate PR. Unfortunately, I'm unable to re-run it at this time.