don't build apphost in cli when running in extension

[adamint]

Description

Isolates the CLI change from #12568, because this behavior should be backported to 13. The apphost should always be built in the extension - the extension can choose whether to call dotnet itself or use a registered task provider like c# dev kit.

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-docs issue (consider using one of the following templates):
      • New (or update) doc-idea template
      • New breaking-change template
      • New diagnostic template
  • 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/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 12621

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 12621"

[Copilot]

Pull Request Overview

This PR simplifies the build skipping logic in the RunCommand by removing the capability-based check and always skipping the build when running in an extension host context. The change consolidates two previous conditions (extension host with DevKit capability OR single-file apphost) into a simpler approach where any extension host scenario skips the build, and removes the now-unused ShouldBuildAppHostInExtensionAsync helper method.

Key changes:

• Simplified build condition from !isSingleFileAppHost || isExtensionHost to !isSingleFileAppHost && !isExtensionHost
• Removed capability check (KnownCapabilities.DevKit) for extension hosts
• Removed ShouldBuildAppHostInExtensionAsync helper method and related logic
• Simplified status message display to always show "Building app host" instead of conditionally showing "Connecting to app host"

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/Aspire.Cli/Commands/RunCommand.cs	Simplified build skipping logic to unconditionally skip builds when running in extension host, removed DevKit capability check, and deleted unused helper method
tests/Aspire.Cli.Tests/Commands/RunCommandTests.cs	Added test case to verify build is skipped when running in extension without requiring DevKit capability

[adamint]

/backport to release/13.0

[github-actions]

Started backporting to release/13.0: https://github.com/dotnet/aspire/actions/runs/19045123423