fix(toolbar): make blockquote toggle range-aware and atomic

[he666-ux]

Summary / 摘要

Make toggleBlockquote() range-aware and atomic so blockquote toggling behaves consistently with existing list toggles.

让 toggleBlockquote() 支持范围选区和原子更新，使 blockquote 的切换行为与现有列表切换保持一致。

Motivation / 背景与动机

toggleBlockquote() previously only operated on the active line and updated the document/selection separately. That made its behavior inconsistent with toggleOrderedList() and toggleUnorderedList(), which already support multi-line ranges and atomic undo.

之前的 toggleBlockquote() 只处理当前行，并且分开更新文档和选区，这与已经支持多行选区和原子撤销的 toggleOrderedList() / toggleUnorderedList() 不一致。

What Changed / 改动内容

• make toggleBlockquote() apply to every covered line in the current selection

• remove > from all selected lines when they are already quoted

• preserve the existing single-line behavior for collapsed selections

• support reversed selections and range boundaries consistently with other block-level toggles

• switch the implementation to a single atomic range replacement so one undo restores both the document and the selection

• 让 toggleBlockquote() 对当前选区覆盖的所有行生效

• 当所有选中行都已带 > 时，统一移除前缀

• 保留折叠光标场景下的单行行为

• 让反向选区和边界行为与其他块级 toggle 保持一致

• 改为单次原子范围替换，保证一次 undo 同时恢复文档和选区

Tests / 测试

• pnpm test packages/plugin-toolbar/test/plugin-toolbar.test.ts
• pnpm test
• pnpm --filter @floatboat/nexus-plugin-toolbar build

Added coverage for:

• single-line blockquote toggle
• multi-line range toggle
• mixed-state, reversed-selection, and boundary behavior
• atomic undo / redo regression cases

新增覆盖包括：

• 单行 blockquote 切换
• 多行范围切换
• 混合状态、反向选区、边界场景
• 原子 undo / redo 回归用例

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