fix: 插件外部dll授权弹窗逻辑优化

[2-2-3-trimethylpentane]

本pr代码为AI生成，我已初步审查确认没有问题。
以下为修复总结：

问题根因

外部插件引用多个 DLL 时，PluginLoadContext 的 Load / LoadUnmanagedDll 对每个依赖 DLL 逐个调用 RequestExternalAuthorization，而原实现存在三个问题：

1. 逐 DLL 弹窗：同一插件的 N 个依赖就弹 N 次确认框
2. 无并发去重：_authorizations 是普通 Dictionary，无锁、无 in-flight 去重，并发解析时同一 DLL 可能弹多个框
3. 非 UI 线程创建窗口：MessageBox.Show 未切 Dispatcher，后台线程创建无 owner 窗口，与启动期工具栏重建队列交错导致渲染错乱
4. 工具栏重复重建：RegisterBoardToolbarItem 每注册一个组件就排队一次 RebuildBoardToolbar，多个组件 = 多次清空/重建控件

修复内容

[PluginAuthorizationService.cs](./Ink Canvas/Plugins/PluginAuthorizationService.cs)

• 串行锁 SemaphoreSlim：同一时刻只允许一个授权对话框
• 会话级决定缓存 BeginSession/EndSession：一次加载会话内，同一插件的首次"允许/拒绝"复用到该插件其余 DLL，不再逐个弹窗；多文件时在消息中列出完整清单
• Dispatcher 切换：所有 MessageBox.Show 强制切到 Application.Current.Dispatcher，并传入 MainWindow 作为 owner
• 持久化授权线程安全：_authorizations 读写加锁

[PluginManager.cs](./Ink Canvas/Plugins/PluginManager.cs)

• LoadAllAsync 和 InstallPendingPackages 加载循环外包裹 BeginSession/EndSession（finally 保证释放）
• 新增 _isLoadingBatch 标志：批量加载期间 RegisterBoardToolbarItem 不逐项排队重建，只设 _boardToolbarRebuildPending
• 新增 FlushPendingBoardToolbarRebuild：加载结束后统一排队一次 RebuildBoardToolbar

[augmentcode]

🤖 Augment PR Summary

Summary: This PR refines external-DLL authorization during plugin loading.

Changes:

• Adds a semaphore to serialize authorization dialogs.
• Adds begin/end loading sessions and caches a per-plugin allow/deny decision.
• Persists hashes for additional DLLs allowed within a session.
• Moves authorization dialogs onto the WPF application dispatcher.
• Uses the loaded main window as the dialog owner when available.
• Protects authorization-dictionary lookup and writes with a lock.
• Wraps startup and hot-install loading loops in authorization sessions.
• Batches whiteboard-toolbar rebuild requests during plugin loading.
• Flushes one deferred rebuild after normal startup loading.

Technical notes: The change aims to prevent repeated dialogs and toolbar reconstruction from interleaving with modal UI during plugin initialization.

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[augmentcode]

Review completed. 2 suggestions posted.

Comment augment review to trigger a new review at any time.

[augmentcode]

Once the first dependency is approved, normal ALC resolution is still blocked in that call, so later dependencies have not yet been added to _sessionPendingFiles or shown in the dialog. This branch then silently persists and loads those unlisted DLLs, so the single confirmation does not disclose the full set of assemblies being authorized.

Severity: medium

Other Locations

• Ink Canvas/Plugins/PluginAuthorizationService.cs:134

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[augmentcode]

PersistAuthorization releases _authorizations before Save, while a session-cached request can enter this method concurrently with the gate-holder. JsonSerializer.Serialize(_authorizations) may enumerate while the other call mutates the dictionary; the swallowed exception then leaves one or more grants unsaved and causes later re-prompts.

Severity: medium

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