Replace stub approve_promotion and reject_promotion gRPC handlers with
real implementations that call PromotionManager.approve() and .reject().
- approve_promotion: removes from pending, promotes to active model map
- reject_promotion: removes from pending with operator-supplied reason
- Input validation: empty model_id returns INVALID_ARGUMENT
- Error handling: nonexistent model_id returns success=false with message
- Add 4 tests covering approve, reject, and not-found error paths
- Add #[cfg(test)] active_models_for_test() accessor on PromotionManager
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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