Add devcontainer.json to support .NET 9 samples in GitHub Codespaces

[Copilot]

Sample projects in this repository that target .NET 9 (such as EverythingServer, AspNetCoreMcpServer, ProtectedMcpServer, etc.) were failing to build or run in GitHub Codespaces with the error:

The command could not be loaded, possibly because:
* You intended to execute a .NET SDK command:
A compatible .NET SDK was not found.
Requested SDK version: 9.0.204
global.json file: /workspaces/csharp-sdk/global.json
Installed SDKs:
8.0.412 [/usr/share/dotnet/sdk]
Install the [9.0.204] .NET SDK or update [/workspaces/csharp-sdk/global.json] to match an installed SDK.

The root cause was that GitHub Codespaces' default container includes .NET 8 LTS but not .NET 9 STS, while the repository's global.json specifies .NET 9.0.204 and several sample projects target net9.0.

This adds a .devcontainer/devcontainer.json configuration that:

• Uses the official Microsoft .NET 8 devcontainer as the base image
• Installs .NET 9.0 SDK using the official devcontainer features (base image already provides .NET 8.0 SDK)
• Includes Node.js to maintain parity with the default GitHub Codespace image
• Configures VS Code with essential C# extensions and sets the default solution file
• Provides a clear confirmation that both SDKs are available after container creation

Now developers can open the repository in GitHub Codespaces and immediately run any sample project without manual SDK installation:

cd samples/EverythingServer
dotnet run # ✅ Works!

All existing .NET 8 samples continue to work normally, ensuring backward compatibility.

Fixes#776.

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[Copilot]

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[eiriktsarpalis]

Tested the codespace and it appears to be working now.

[mikekistler]

This looks good but I think there are a couple small improvements we should make.

[mikekistler]

Looks good! 👍