Create .aspire/settings.json with appHostPath during aspire new

[Copilot]

Running aspire new creates a project but no .aspire/settings.json file, causing the VS Code extension to immediately prompt for appHostPath configuration after opening the project.

Changes

Modified DotNetTemplateFactory.ApplyTemplateAsync to create .aspire/settings.json after successful template instantiation:

• Added CreateSettingsFileAsync method that:
  • Searches output directory for AppHost projects (both .csproj and single-file apphost.cs)
  • Validates AppHost projects using existing GetAppHostInformationAsync infrastructure
  • Creates .aspire/settings.json with relative path to AppHost project
  • Uses JsonSourceGenerationContext for formatted output
  • Gracefully handles failures without breaking template creation

Example

Running aspire new aspire-starter --name MyApp --output ./myapp now creates:

./myapp/
├── .aspire/
│   └── settings.json          # New: {"appHostPath": "../MyApp.AppHost/MyApp.AppHost.csproj"}
├── MyApp.AppHost/
│   └── MyApp.AppHost.csproj
├── MyApp.Web/
└── MyApp.sln

Original prompt

---

This section details on the original issue you should resolve

<issue_title>aspire new should create a .aspire/settings.json file with appHostPath set after creation</issue_title>
<issue_description>This impacts the aspire extension as it will prompt you to set appHostPath immediately after opening the project</issue_description>

<agent_instructions>When running aspire new, if there is no existing appHostPath configuration setting the .aspire/settings.json file. If the file is created as part of aspire new then make sure that it is placed in the output directory and that the appHostPath is pointing to the right location relative to the location of the settings file.

It may be necessary to extract the existing logic that generates the appHostPath based on the working directory into a helper so that the base path can be overridden.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes aspire new should create a .aspire/settings.json file with appHostPath set after creation #12523

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[github-actions]

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