Moving property IsAspireHost to the SDK

[joperezr]

Description

When we introduced the Aspire SDK, we tried to minimize the changes to the AppHost templates for backward compatibility, which meant that the definition of IsAspireHost stayed in the AppHost file. This could sometimes cause issues when restore wouldn't succeed the first time (like #7977) so this change is finally moving that property from the AppHost and into the SDK to avoid those types of issues.

cc: @DamianEdwards

Fixes #7977
Fixes #6142

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
• Are you including unit tests for the changes and scenario tests if relevant?
  • No
• Did you add public API?
  • No
• Does the change make any security assumptions or guarantees?
  • No

Tracking docs update issue: dotnet/docs-aspire#2847

[Copilot]

Copilot wasn't able to review any files in this pull request.

Files not reviewed (4)

• src/Aspire.AppHost.Sdk/SDK/Sdk.in.props: Language not supported
• src/Aspire.ProjectTemplates/templates/aspire-apphost/9.2/Aspire.AppHost1.csproj: Language not supported
• src/Aspire.ProjectTemplates/templates/aspire-empty/9.2/AspireApplication.1.AppHost/AspireApplication.1.AppHost.csproj: Language not supported
• src/Aspire.ProjectTemplates/templates/aspire-starter/9.2/Aspire-StarterApplication.1.AppHost/Aspire-StarterApplication.1.AppHost.csproj: Language not supported

[DamianEdwards]

What does this property actually do now then? What is looking at it?

[joperezr]

What does this property actually do now then? What is looking at it?

Several things in our apphost targets as well as sdk targets use it to drive some of their functionality. https://github.com/search?q=repo%3Adotnet%2Faspire%20%22%24(IsAspireHost)%22&type=code

That can get refactored to not use the property, but some scenarios might get a bit messy and would require some logic that might be a bit more complex. For instance, if you have a project that references an AppHost, it imports some of these props and targets and so now we would need to make sure that we condition the logic right such that it doesn't start behaving like an AppHost.