undo() fails silently for LoroCounter operations after the document receives changes from a remote peer

[canadaduane]

This only fails for LoroCounter, other containers (such as LoroText) work fine:

    const docA = new LoroDoc()
    const docB = new LoroDoc()

    docA.setPeerId("1")
    docB.setPeerId("2")

    const undoA = new UndoManager(docA, { mergeInterval: 0 })
    const counterA = docA.getCounter("priority")
    const counterB = docB.getCounter("priority")

    // Step 1: A increments counter (0 → 1)
    counterA.increment(1)
    docA.commit()
    expect(counterA.value).toBe(1)
    expect(undoA.canUndo()).toBe(true)

    // Step 2: Sync A → B
    docB.import(docA.export({ mode: "update" }))
    expect(counterB.value).toBe(1)

    // Step 3: B increments counter (1 → 2)
    counterB.increment(1)
    docB.commit()
    expect(counterB.value).toBe(2)

    // Step 4: Sync B → A (A now has remote changes)
    docA.import(docB.export({ mode: "update" }))
    expect(counterA.value).toBe(2)
    expect(undoA.canUndo()).toBe(true)

    // Step 5: A tries to undo their increment
    const undoResult = undoA.undo()
    docA.commit()

    // BUG: undo returns true but value doesn't change
    console.log("undo() returned:", undoResult)
    console.log("Counter value after undo:", counterA.value)

    // Expected: A's increment is undone, leaving B's increment (value = 1)
    // Actual: Value stays at 2
    expect(counterA.value).toBe(1) // This fails!

[canadaduane]

Fixed by #906