Add installation validation with user notifications for JavaScript package managers

[Copilot]

Description

Adds installation validation for Node.js and JavaScript package managers (npm, yarn, pnpm) following the same pattern from #12787 for Python/uv. When commands are missing, users see notifications with installation links rather than cryptic errors.

Shared Infrastructure

• Added CoalescingAsyncOperation.cs and RequiredCommandValidator.cs references from src/Shared/ as internal shared source
• Compiled into project via <Compile Include="$(SharedDir)..."> references

Validation Managers

Created four installation managers extending RequiredCommandValidator:

• NodeInstallationManager - validates node command
• NpmInstallationManager - validates npm command
• YarnInstallationManager - validates yarn command
• PnpmInstallationManager - validates pnpm command

Each provides EnsureInstalledAsync(bool throwOnFailure = true) and help links to installation docs.

Integration Points

Service Registration:

• NodeInstallationManager in: AddNodeApp, AddJavaScriptApp, AddViteApp
• Package manager validators in: WithNpm, WithYarn, WithPnpm

Validation Hooks:

• Added OnBeforeResourceStarted hook on JavaScriptInstallerResource
• Node validation runs first (all package managers require Node)
• Package manager validation runs based on configured manager (npm/yarn/pnpm)
• Uses throwOnFailure: false to show notifications without altering natural error flow

Behavior

When a command is missing:

1. Warning logged
2. Notification shown via IInteractionService with installation link
3. App continues startup and fails naturally with original error
4. Existing error behavior preserved

Example notification:

Missing command: node
Required command 'node' was not found. See installation instructions for more details.
[Installation instructions] → https://nodejs.org/en/download/

Checklist

• Is this feature complete?
  • [x] Yes. Ready to ship.
  • [ ] No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • [x] Yes
  • [ ] No
• Did you add public API?
  • [ ] Yes
  • [x] No
• Does the change make any security assumptions or guarantees?
  • [ ] Yes
  • [x] No
• Does the change require an update in our Aspire docs?
  • [ ] Yes
  • [x] No

Original prompt

Make the same change in https://github.com/dotnet/aspire/pull/12787 but for Aspire.Hosting.JavaScript. yarn, npm, pnpm and node.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[github-actions[bot]]

🚀 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 -- 12840

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 12840"