fix(ticket-assistant): rendre visible l'échec « aucune réponse » de l'assistant de ticket (#60)

Introduit un variant terminal `ReplyEvent::Error { message }` (domain/ports)
propagé jusqu'au chat : l'adapter OpenAI-compat récupère `reasoning_content`
quand `content` est vide, et un `Final` vide est converti en `Error` visible
plutôt qu'un tour silencieux qui pend. Le front (`ReplyChunk` + useTicketAssistant)
rend ce message d'erreur dans la cellule.

- domain: variant `ReplyEvent::Error`, bras non terminal (readiness, structured drain)
- infra/session: parse `reasoning_content` (delta/completion), fallback visible
- app-tauri: mapping ReplyEvent::Error -> ReplyChunk::Error, Final vide -> Error
- frontend: ReplyChunk `error`, rendu dans useTicketAssistant (+ test .test.tsx)

NB: commit sur la feature branch, PAS un merge. #60 reste inProgress.
Les 10 tests `cargo test -p infrastructure --lib session` échouent SOUS SANDBOX
uniquement (bind loopback 127.0.0.1:0 interdit -> Os PermissionDenied), pas une
régression : revérification hors-sandbox requise avant tout merge vers develop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/app-tauri/src/dto.rs

@@ -1745,10 +1745,11 @@ impl From<TerminalSession> for TerminalSessionDto {
 /// twin of a [`domain::ports::ReplyEvent`]: the `agent_send` pump maps each turn
 /// event to one of these and pushes it to the frontend `AgentChatView`.
 ///
-/// Tagged on `kind` (camelCase: `"textDelta"` | `"toolActivity"` | `"final"`), so
-/// the front branches without positional parsing. `Final` is the **deterministic
-/// terminal** chunk of a turn — after it the turn is frozen and the stream ends.
-/// `Deserialize` is derived too so tests (and a mock gateway) can round-trip it.
+/// Tagged on `kind` (camelCase: `"textDelta"` | `"toolActivity"` | `"final"` |
+/// `"error"`), so the front branches without positional parsing. `Final` is the
+/// normal deterministic terminal chunk of a turn; `Error` is a visible terminal
+/// fallback for an otherwise silent/empty turn. `Deserialize` is derived too so
+/// tests (and a mock gateway) can round-trip it.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(tag = "kind", rename_all = "camelCase")]
 pub enum ReplyChunk {
@@ -1770,6 +1771,12 @@ pub enum ReplyChunk {
         /// The aggregated final content of the turn.
         content: String,
     },
+    /// A visible terminal fallback when the model stream produced no usable answer.
+    #[serde(rename_all = "camelCase")]
+    Error {
+        /// Human-readable explanation to render in the chat cell.
+        message: String,
+    },
 }
 
 /// Response DTO for `reattach_agent_chat`: the retained **conversation