Improve missing npm install messaging

[IEvangelist]

Description

Improve CLI messaging when guest-language project creation cannot run npm install because Node.js tools are missing.

Before this change, the CLI surfaced the raw missing npm/npx tool output or treated dependency installation as a failure even after project files had already been created. That made the TypeScript/guest-language scaffolding experience look broken or incomplete, even though the main issue was simply that Node.js was not available on PATH.

This change adds a small matcher for the existing missing-tool output and uses it to convert that condition into a warning in the flows where project creation has already succeeded. The warning explains that the files were created, automatic npm install could not run, and the user should install Node.js and run npm install manually.

Validation:

• Added unit tests for the warning matcher
• Ran focused CLI tests: CommandPathResolverTests and AutomaticNpmInstallWarningTests (10 passed)

Dependencies: None.

Fixes #15420

Example warning output

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 -- 15732

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 15732"

[Copilot]

Pull request overview

Improves Aspire CLI UX when guest-language scaffolding can’t run npm install due to missing Node.js tooling by detecting the known “missing npm/npx” output and downgrading that case to a warning (when project creation already succeeded).

Changes:

• Added AutomaticNpmInstallWarning matcher + standardized warning message for missing npm/npx.
• Updated guest-language scaffolding and SDK codegen flows to treat missing Node tools as a warning in “best-effort” dependency installation paths.
• Added unit tests covering the matcher and the warning message text.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/Aspire.Cli.Tests/Utils/AutomaticNpmInstallWarningTests.cs	Adds unit tests for missing-tool output matching and the warning message.
src/Aspire.Cli/Utils/AutomaticNpmInstallWarning.cs	Introduces a small matcher + warning message constant for missing npm/npx.
src/Aspire.Cli/Scaffolding/ScaffoldingService.cs	Converts missing Node tools during guest-language scaffolding dependency install into a warning and continues.
src/Aspire.Cli/Projects/GuestAppHostProject.cs	Adds an option to treat missing Node tools as a warning in the “best effort” dependency install after SDK codegen.

[github-actions]

Re-running the failed jobs in the CI workflow for this pull request because 1 job was identified as retry-safe transient failures in the CI run attempt.
GitHub was asked to rerun all failed jobs for that attempt, and the rerun is being tracked in the rerun attempt.
The job links below point to the failed attempt jobs that matched the retry-safe transient failure rules.

• Tests / Hosting-1 / Hosting-1 (windows-latest) - Job-level runner or infrastructure failure matched the transient allowlist.