[lexical] Bug Fix: insert at the caret inside an inline ElementNode that cannot be split

[LeSingh1]

RangeSelection.insertNodes splits the ancestor chain from the anchor up to the nearest block before splicing inline content in. An ElementNode is split by moving the children after the caret into the node returned by its insertNewAfter(), which returns null for any ElementNode that does not implement it — the base class default. $splitNodeAtPoint ignored that null and still reported the position after the whole node, so content pasted with the caret inside a custom inline ElementNode landed after the node while typing the same text inserted it in place.

Stop the split walk on an unsplittable inline element and splice into it at the caret.

Nodes that implement insertNewAfter — LinkNode, MarkNode, ListItemNode, CodeNode — are unaffected and still split. $removeTextAndSplitBlock now returns the container alongside the index, but only the one affected call site opts into the new walk; insertParagraph and the code-block and slot paste paths take the index alone and are unchanged.

Reverting the product file fails 2 of the 3 new tests with the reported symptom — text("abcd")) text("XYZ") instead of text("ab") text("XYZ") text("cd"). The third is a control.

Fixes #6477

[vercel]

@LeSingh1 is attempting to deploy a commit to the Meta Open Source Team on Vercel.

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[LeSingh1]

Consolidated into #9058 with the other PRs that share this defect, per @etrepum's note on #9027 and @mayrang's on #9035. Same fix and same tests, one review.