[lexical-yjs][lexical-react] Bug Fix: collab initialEditorState is no longer undoable

[LeSingh1]

When CollaborationPlugin bootstraps an empty shared document it writes initialEditorState through a normal editor update, which syncs into Yjs with the binding as the transaction origin. createUndoManager tracks that origin, so the bootstrap becomes the first undo stack item and the very first undo wipes the initial content. A non-collab editor applies its initial state with HISTORY_MERGE_TAG and it is never undoable, so collab behaved inconsistently — and content that is meant to be fixed layout can be removed entirely by the user.

This adds an isBootstrapping flag to the binding and has createUndoManager pass a captureTransaction that skips transactions produced while it is set. Both the V1 and V2 bootstrap paths go through a shared bootstrapEditor helper that raises it.

The flag is cleared from a microtask rather than a finally, because editor.update commits — and therefore syncs to Yjs — in a microtask that Lexical has already queued. Clearing it there rather than from the commit also means it cannot get stuck if the bootstrap update is a no-op.

For the record I first tried discrete: true on the bootstrap update instead. It fixed the new test but broke 19 existing collab tests by changing bootstrap interleaving, so I dropped that approach.

Adds a unit test that bootstraps an initial state and then undoes; it fails on main.

Fixes #7110

[vercel]

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

A member of the Team first needs to authorize it.

[LeSingh1]

Consolidated into #9057 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.