- Introduced `FrontendAuthProviderExclusive` capability to restrict authentication to a single selected provider.
- Added `SetExclusiveProvider` and `ClearExclusiveProvider` methods for managing exclusive providers in the access registry.
- Updated `pluginhost` to prioritize and enforce exclusive providers based on plugin priority and ID.
- Enhanced RPC capabilities schema to include `FrontendAuthProviderExclusive` field.
- Added example plugin and tests for exclusive frontend auth behavior.