- Add reject flags and reason fields to `SchedulerPickResponse` with snake_case JSON unmarshaling support
- Terminate candidate selection with an auth error when scheduler rejection is flagged
- Normalize rejection response with default error code and reason in the plugin host
Closes: #6130
- Add `SchedulerAcrossPriorities` capability to allow plugin schedulers to receive candidates across all priority tiers.
- Propagate scheduler priority preferences through the plugin host and RPC capabilities.
- Update auth manager selection logic to supply candidates across all priorities when the scheduler opts in.
Closes: #5894
- Added a Go scheduler plugin demonstrating CLIProxyAPI capabilities, such as `plugin.register`, `plugin.reconfigure`, and `scheduler.pick`.
- Implemented methods for plugin configuration, built-in scheduler delegation (`fill-first`, `round-robin`), dynamic candidate selection, and error handling.
- Extended `pluginhost` with scheduler handling, candidate normalization, and fallback mechanisms.
- Included examples, tests, and detailed documentation for scheduler usage and implementation.