Test team scoping of plugin UI views in multi-team mode

[o-nikolas]

Plugin nav items, external views, and React apps are served to the UI through GET /api/v2/plugins, which already filters plugins by the user's authorized teams when multi-team mode is enabled. Because external_views and react_apps are nested in each plugin's response, they are scoped along with their plugin — a team-scoped plugin's views and apps are never sent to users outside that team.

That guarantee had no direct test coverage (existing tests used plugins without views). This adds TestGetPluginsUIViewTeamFiltering, which verifies that a user sees only their teams' (plus global) external views and React apps, that a user with no teams sees only global ones, and that all views are returned when multi-team mode is disabled.

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