Run VSIX packaging validation for extension dependency PRs

[Copilot]

VSIX dependency updates previously bypassed packaging in PR validation, allowing invalid extension dependency graphs to reach release packaging. This adds a credential-free packaging check for changed VS Code extension manifests.

• PR VSIX pack stage

  • Adds a PR-only VSIXPack stage after build completion.
  • Detects changes to servers/*/vscode/package.json and package-lock.json, including deletions.

• Credential-free packaging

  • Downloads existing unsigned build artifacts.
  • Consolidates artifacts without signing preparation.
  • Runs Pack-Vsix.ps1, including npm ci --omit=optional and vsce package.
  • Does not authenticate to npm feeds, sign, publish, or upload VSIX artifacts.

• Artifact preparation

  • Adds -PrepareForVsix to Compress-ForSigning.ps1 so PR packaging preserves macOS binaries and supports multi-server build artifacts.

• Documentation

  • Notes the conditional VSIX packaging validation in CONTRIBUTING.md.

condition: and(succeeded(), eq(variables['Build.Reason'], 'PullRequest'))

• Fixes Run VSIX packaging on dependency-update PRs to catch quarantine failures before merge #3571

[azure-pipelines]

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