Update Yarn usage to Yarn 4

[adamint]

Description

Updates the repository's Yarn usage from Yarn Classic to Yarn 4.

Summary:

• Installs Yarn 4.14.1 through Corepack in official, unofficial, and CodeQL pipelines, including an explicit corepack@latest install.
• Bridges npmAuthenticate@0 credentials into Yarn 4 auth environment variables so the dnceng npm feed remains usable, and adds the missing npm auth setup to CodeQL because it builds the VS Code extension.
• Migrates the VS Code extension from .yarnrc to .yarnrc.yml, pins packageManager to yarn@4.14.1, and regenerates extension/yarn.lock in Yarn 4 format.
• Updates extension build helpers/docs to use Corepack and removes the orphan root Yarn Classic lockfile.

Validation:

• YARN_NPM_REGISTRY_SERVER=https://registry.npmjs.org YARN_NPM_ALWAYS_AUTH=false yarn install --immutable from extension/ passed with existing peer dependency warnings.
• git diff --check passed.
• VS Code diagnostics show no errors in azure-pipelines-codeql.yml, azure-pipelines-unofficial.yml, or extension/.yarnrc.yml; existing diagnostics remain in azure-pipelines.yml and extension/package.json.

Fixes # (issue)

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 -- 16781

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 16781"

[Copilot]

Pull request overview

This PR migrates the repo’s VS Code extension and CI pipelines from Yarn Classic (v1) to Yarn 4, standardizing Yarn installation/activation via Corepack and updating config/lockfiles to the Yarn Berry format.

Changes:

• Install/activate yarn@4.14.1 via Corepack in official, unofficial, and CodeQL Azure Pipelines.
• Update the VS Code extension to use Yarn 4 conventions (packageManager, .yarnrc.yml) and remove the legacy .yarnrc and root Yarn Classic lockfile.
• Add repo ignores for Yarn Berry artifacts and update extension build helpers/docs to reference Corepack.

Reviewed changes

Copilot reviewed 12 out of 15 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
yarn.lock	Removes the root Yarn Classic (v1) lockfile.
extension/package.json	Pins the package manager to yarn@4.14.1.
extension/Extension.proj	Attempts to enable Yarn via Corepack before version checks during extension build.
extension/CONTRIBUTING.MD	Updates contributor prerequisites to Corepack + pinned Yarn.
extension/build.sh	Enables Yarn via Corepack when Yarn isn’t present; updates install guidance.
extension/build.ps1	Enables Yarn via Corepack when Yarn isn’t present; updates install guidance.
extension/.yarnrc.yml	Introduces Yarn 4 config (node-modules linker, registry/auth settings).
extension/.yarnrc	Removes legacy Yarn Classic configuration.
extension/.vscodeignore	Excludes Yarn 4 config/artifacts from packaged VSIX.
eng/pipelines/azure-pipelines.yml	Switches CI Yarn installation to Corepack + Yarn 4 and bridges npm auth into Yarn env vars.
eng/pipelines/azure-pipelines-unofficial.yml	Same Yarn/Corepack + auth bridging updates for unofficial pipeline.
eng/pipelines/azure-pipelines-codeql.yml	Adds npm auth + Yarn/Corepack setup so CodeQL can build the extension.
.gitignore	Ignores Yarn Berry artifacts (.yarn/, .pnp.*).
.github/workflows/tests.yml	Updates GitHub Actions test workflow to set up Yarn (per PR diff).