chore: 启用 .NET 预览特性并开启 runtime-async 功能

[joesdu]

📝 描述 (Description)

新增 PropertyGroup，设置 EnablePreviewFeatures 为 true，并在 Features 属性中启用 runtime-async=on，以便项目可使用 .NET 预览版中的异步运行时相关新特性。

🔗 相关问题 (Related Issues)

• Closes #
• Fixes #

🏷️ 变更类型 (Type of Change)

• 🐛 错误修复 (Bug fix) (non-breaking change which fixes an issue)
• ✨ 新功能 (New feature) (non-breaking change which adds functionality)
• 💥 破坏性变更 (Breaking change) (fix or feature that would cause existing functionality to not work as expected)
• 📚 文档更新 (Documentation update)
• 🔧 重构 (Refactor) (no functional changes)
• 🧪 测试 (Tests) (adding or updating tests)
• 🛠️ 构建/CI 变更 (Build/CI changes)
• 🎨 样式变更 (Style changes) (formatting, etc.)
• 🚀 性能改进 (Performance improvements)
• 🔒 安全改进 (Security improvements)

✅ 检查清单 (Checklist)

• 我的代码遵循项目的编码标准 (My code follows the project's coding standards)
• 我已经对自己的代码进行了自我审查 (I have performed a self-review of my own code)
• 我已经在难以理解的区域注释了我的代码 (I have commented my code, particularly in hard-to-understand areas)
• 我已经对文档进行了相应的更改 (I have made corresponding changes to the documentation)
• 我的更改不会产生新的警告或错误 (My changes generate no new warnings or errors)
• 我已经添加了证明我的修复有效或我的功能工作的测试 (I have added tests that prove my fix is effective or that my feature works)
• 新增和现有的单元测试在本地通过我的更改 (New and existing unit tests pass locally with my changes)
• 任何依赖变更都已合并并发布到下游模块 (Any dependent changes have been merged and published in downstream modules)

🧪 测试 (Testing)

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📸 截图 (如果适用) (Screenshots (if applicable))

🔍 附加说明 (Additional Notes)

[Copilot]

Pull request overview

Summary / 总结:

This PR adds preview features configuration to the root Directory.Build.props file by setting EnablePreviewFeatures=true and enabling runtime-async=on feature flag. While the intent aligns with the project's stated use of preview features, the implementation has a critical issue: enabling preview features in library projects that produce NuGet packages will cause NETSDK1138 warnings for all package consumers.

该 PR 在根级 Directory.Build.props 文件中添加预览特性配置,设置 EnablePreviewFeatures=true 并启用 runtime-async=on 功能标志。虽然意图与项目声明使用预览特性相符,但实现存在关键问题:在生成 NuGet 包的库项目中启用预览特性会导致所有包使用者收到 NETSDK1138 警告。

Changes / 变更:

• Added EnablePreviewFeatures=true to enable .NET preview runtime/BCL APIs
• Added Features=$(Features);runtime-async=on to enable async runtime optimizations

Key Issues / 主要问题:

• Setting EnablePreviewFeatures=true in library projects causes NETSDK1138 warnings for NuGet package consumers
• The project already has LangVersion=preview which provides access to preview C# language features without the consumer-facing warnings
• No documentation updates explain the impact on package consumers or the rationale for requiring preview runtime APIs

建议 / Suggested Changes:

• Determine if preview runtime/BCL APIs (marked with [RequiresPreviewFeatures]) are actually needed
• If only preview C# language features are needed, remove EnablePreviewFeatures=true (keep LangVersion=preview)
• If preview runtime APIs are essential, add clear documentation and consider SuppressNETCoreSdkPreviewMessage=true
• The runtime-async=on feature can be kept independently