Centralize remote plugin mutations in PluginsManager

[copyberry]

Centralize remote plugin mutations in PluginsManager

What changed

• Move remote plugin install and uninstall orchestration from the app server into shared PluginsManager APIs.
• Keep cache and backend mutations coordinated by the installed-plugin sync gate, and retain install outcomes long enough to protect newly materialized bundles during downstream setup.
• Return structured operation errors and outcomes so callers can preserve JSON-RPC error mapping, telemetry, cache refreshes, and OAuth setup.

Testing

• Add regression coverage that verifies uninstall holds the mutation gate, preserves the local cache when the backend operation fails, and refreshes installed state after a successful uninstall.