Expose plugin installation interstitial requirements

[copyberry]

Expose plugin installation interstitial requirements

What changed

• Add nullable mustShowInstallationInterstitial metadata to app-server
  PluginSummary responses.
• Preserve the remote service value through catalog, installed-plugin cache,
  list, read, and share-list paths.
• Return null for local plugins and remote responses that omit the field so
  clients can fail closed when the policy is unavailable.

Testing

• Cover true, false, and missing values across remote summary paths and
  app-server plugin endpoints.

[github-actions]

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