feat(chat)!: add ToolStepOutcome::Stop handling and notification events channel

- Add handling for `ToolStepOutcome::Stop` in chat tests (treat as terminal state)
- Initialize `notification_events_tx` channel in GlobalContext with capacity 256

BREAKING CHANGE: Introduces new `ToolStepOutcome::Stop` variant which terminates
tool execution loops (previously unhandled).

Author: JegernOUTT

refact-agent/engine/src/chat/tests.rs

@@ -1180,6 +1180,7 @@ mod tests {
                 ToolStepOutcome::NoToolCalls => false,
                 ToolStepOutcome::Paused => false,
                 ToolStepOutcome::Continue => true,
+                ToolStepOutcome::Stop => false,
             }
         }
 
@@ -1205,6 +1206,7 @@ mod tests {
                     ToolStepOutcome::NoToolCalls => break,
                     ToolStepOutcome::Paused => break,
                     ToolStepOutcome::Continue => continue,
+                    ToolStepOutcome::Stop => break,
                 }
             }
             cycles

refact-agent/engine/src/global_context.rs

@@ -685,6 +685,7 @@ pub mod tests {
             trajectory_events_tx: Some(tokio::sync::broadcast::channel(1024).0),
             task_events_tx: Some(tokio::sync::broadcast::channel(1024).0),
             task_events_seq: Some(Arc::new(std::sync::atomic::AtomicU64::new(0))),
+            notification_events_tx: Some(tokio::sync::broadcast::channel(256).0),
             chat_sessions: crate::chat::create_sessions_map(),
             voice_service: crate::voice::VoiceService::new(),
             project_registry_cache: Arc::new(StdRwLock::new(RegistryCacheManager::new())),