Fix crash restoring a session serialized without undo history

[xyos]

Problem

Restoring a session with EditSession.fromJSON() crashes on the first subsequent edit when the serialized session was using the default no-op undo manager:

Uncaught TypeError: Cannot set properties of undefined (setting 'length')
    at UndoManager.add (undomanager.js:45:58)
    at EditSession.onChange (edit_session.js:303:31)

EditSession.toJSON() serializes the undo manager under history; a session that never had a real UndoManager attached serializes it as {}. On restore, fromJSON then does:

undoManager.$undoStack = session.history.$undoStack; // undefined
undoManager.$redoStack = session.history.$redoStack; // undefined

clobbering the arrays the UndoManager constructor just created, so the first UndoManager.add() throws at this.$redoStack.length = 0.

Repro

var session = new EditSession("hello");            // default no-op undo manager
var restored = EditSession.fromJSON(JSON.stringify(session.toJSON()));
restored.insert({row: 0, column: 0}, "x");          // TypeError

This is hit in practice by the kitchen-sink demo, which persists the session to localStorage on unload and restores it via fromJSON on the next load — after which every keystroke/paste throws. Regression from the history serialization changes in #5920.

Fix

• EditSession.fromJSON: tolerate a missing/empty history object and default $undoStack/$redoStack to [], mark/rev to 0.
• UndoManager.fromJSON: guard against null/partial input the same way.
• Regression test: round-trip a session without undo history and edit it.

Round-tripping a session with real history is unaffected (covered by the existing JSON serialization preserves undo/redo history test).

Open kitchen-sink @ ae482be47cbf147e7a550f1efc3487e0928f847a

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.74%. Comparing base (f82010e) to head (ae482be).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5989      +/-   ##
==========================================
+ Coverage   93.73%   93.74%   +0.01%     
==========================================
  Files         641      641              
  Lines      138167   138182      +15     
  Branches    14561    14559       -2     
==========================================
+ Hits       129510   129539      +29     
+ Misses       8657     8643      -14     

Flag	Coverage Δ
unittests	93.74% <100.00%> (+0.01%)	⬆️

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