fix(chat): le bouton Stop de la CLI custom arrête l'agent en cascade (#187)

Le Stop du CustomAgentChatView n'annulait que le tour courant sans libérer la
session ni toucher aux tâches enfants ou appels MCP en cours. Il appelle
désormais stop_live_agent sur l'agent vivant, qui:

- ajoute le port StopAgentExecutionCascade (best-effort, idempotent), implémenté
  par OrchestratorService pour annuler les idea_run_in_background rattachés au
  rendez-vous actif (via BackgroundTaskRunner.cancel);
- cascade l'arrêt aux wait-dépendances (stop_turn_watch + preempt) pour
  interrompre les appels MCP et les attentes inter-agents;
- libère et supprime la session structurée.

UI: nouvel état "Arrêt en cours..." (bouton disabled + loading) pendant l'arrêt,
libération du sessionId, label expliquant la cascade demandée. La Tauri command
cancel_agent_chat est alignée sur stop_live_agent + stop_turn_watch agent et
dépendances.

Tests: SpyBackgroundRunner (cascade n'annule que les tâches rattachées),
workstate_actions cascade best-effort, CustomAgentChatView stop+cascade feedback
et fallback NOT_FOUND.

crates/app-tauri/src/commands.rs

@@ -2764,29 +2764,39 @@ pub async fn interrupt_agent(
         .map_err(ErrorDto::from)
 }
 
-/// `cancel_agent_chat` — interrupt the current turn of a live structured chat
-/// session without shutting the session down.
+/// `cancel_agent_chat` — stop a live structured chat session and cascade its
+/// running execution children best-effort.
 ///
-/// This is the session-id based twin of [`interrupt_agent`] for a custom chat
+/// This is the session-id based twin of [`stop_live_agent`] for a custom chat
 /// surface that already owns the structured `sessionId`.
 ///
 /// # Errors
 /// Returns an [`ErrorDto`] (`INVALID` for a malformed id, `NOT_FOUND` if no live
-/// structured session owns the id, `PROCESS` if a concrete cancel fails).
+/// structured session owns the id, `PROCESS` if a concrete stop fails).
 #[tauri::command]
 pub async fn cancel_agent_chat(
     session_id: String,
     state: State<'_, AppState>,
 ) -> Result<(), ErrorDto> {
     let sid = parse_session_id(&session_id)?;
-    let session = state
+    let (project_id, agent_id, _, _) = state
         .structured_sessions
-        .session(&sid)
+        .meta_for_session(&sid)
         .ok_or_else(|| ErrorDto::from(AppError::NotFound(format!("structured session {sid}"))))?;
-    session
-        .cancel_current_turn()
+    let project = resolve_project(&project_id.to_string(), &state).await?;
+    let dependencies = state
+        .orchestrator_service
+        .active_wait_dependencies(agent_id);
+    state.stop_turn_watch(project.id, agent_id);
+    for dependency in dependencies {
+        state.stop_turn_watch(project.id, dependency);
+    }
+    state
+        .stop_live_agent
+        .execute(StopLiveAgentInput { project, agent_id })
         .await
-        .map_err(|e| ErrorDto::from(AppError::from(e)))
+        .map(|_| ())
+        .map_err(ErrorDto::from)
 }
 
 async fn preferred_view_for_agent(