Use C# Dev Kit selected workspace dotnet host for debugger prerequisite checks

### Problem

When C# Dev Kit owns workspace SDK selection through Workspace Requirements, the C# extension still independently runs `dotnet --info` against its own configured paths/ambient `PATH` during debugger activation.

In a Remote SSH Linux workspace with no ambient `dotnet` on `PATH`, this produces two competing remediations at once:

- C# Dev Kit opens Workspace Requirements with **Install .NET 10 SDK**.
- The C# extension shows **The .NET SDK cannot be located** with **Get the SDK**.

The C# notification is especially confusing because its **Get the SDK** action already delegates to `csdevkit.installDotnetSdk` when C# Dev Kit is present, while the prerequisite check that raised it did not use C# Dev Kit's selected host or blocked state.

Observed with:

- C# Dev Kit `9.9.554-g91f1bb`
- C# `2.140.9`
- VS Code Remote SSH to Ubuntu 24.04 x64
- no ambient `dotnet` on the remote `PATH`

Relevant current code:

- `src/coreclrDebug/util.ts`: `checkDotNetCli()` runs `dotnet --info` using C# extension paths.
- `src/coreclrDebug/activate.ts`: `showDotnetToolsWarning()` routes **Get the SDK** to `csdevkit.installDotnetSdk` when Dev Kit is present.
- `src/utils/getCSharpDevKit.ts` and `src/csharpDevKitExports.ts`: C# already discovers and consumes C# Dev Kit exports, but there is no selected workspace host/readiness contract.

### Expected behavior

C# Dev Kit should be the single SDK/host authority when it is active and `dotnet.preferCSharpExtension` is not selected:

1. C# obtains the exact workspace `dotnet` executable and environment selected by C# Dev Kit before debugger prerequisite checks.
2. If Workspace Requirements is blocked and has opened its remediation UI, C# does not show a second SDK-install notification.
3. After the user installs or selects the SDK and reloads, both extensions use the same host.
4. When C# Dev Kit is absent or explicitly bypassed, C# retains its current standalone discovery and notification behavior.

A minimal contract could be an optional C# Dev Kit export returning `{ status: ready | blocked, dotnetPath?, environment? }`. Because workspace host changes are already reload-bound, this does not require live language-server/debugger host switching.

### Related

- microsoft/vscode-dotnettools#1243
- microsoft/vscode-dotnettools#792
- dotnet/vscode-csharp#6620

Start with src/coreclrDebug/util.ts and checkDotNetCli(), then trace src/coreclrDebug/activate.ts and showDotnetToolsWarning(), along with the C# Dev Kit discovery and export files. Reproduce the Remote SSH activation case and verify that C# uses the selected workspace host, suppresses duplicate remediation while blocked, and preserves standalone behavior when Dev Kit is absent or bypassed.

Written by the indexing model from the issue text.