Fix interactive C# aspire init config collision

[adamint]

Description

Fixes interactive aspire init in an empty folder when the user accepts the default C# AppHost language. The language selection is no longer written to local config before the single-file C# template runs, which avoids the aspire.config.json overwrite collision. The single-file AppHost template now records appHost.language so the generated config still has complete AppHost metadata.

Fixes #15750

Validation:

• dotnet test tests/Aspire.Cli.Tests/Aspire.Cli.Tests.csproj -- --filter-class "*.InitCommandTests" --filter-not-trait "quarantined=true" --filter-not-trait "outerloop=true"
• dotnet build tests/Aspire.Cli.EndToEnd.Tests/Aspire.Cli.EndToEnd.Tests.csproj

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire.dev issue:
      • New issue
  • No

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 16447

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 16447"

[Copilot]

Pull request overview

Fixes an aspire init failure in empty folders when accepting the default C# AppHost language by avoiding a premature aspire.config.json write that caused dotnet new to fail due to conflicting files.

Changes:

• Stop persisting the prompted language selection before running the single-file C# template; persist only after successful scaffolding where appropriate.
• Update the aspire-apphost-singlefile template’s aspire.config.json to include appHost.language.
• Add a unit regression test and a new end-to-end test covering the interactive default-C# aspire init flow.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Aspire.Cli/Commands/InitCommand.cs	Defers language persistence to avoid aspire.config.json overwrite collisions during single-file C# init.
src/Aspire.ProjectTemplates/templates/aspire-apphost-singlefile/aspire.config.json	Ensures the single-file AppHost template emits appHost.language in config.
tests/Aspire.Cli.Tests/Commands/InitCommandTests.cs	Adds regression coverage for the collision scenario described in #15750.
tests/Aspire.Cli.EndToEnd.Tests/CSharpInitTests.cs	Adds interactive E2E coverage to ensure default C# init succeeds and outputs expected files/config.

[radical]

Two suggestions from review — both are about making the fix more self-documenting and robust.