Merge feature/ticket60-ticket-assistant-no-reply into develop (#60)

Rendre visible l'échec « aucune réponse » de l'assistant de ticket :
stream sans Final ou Final vide → ReplyChunk::Error terminal visible,
plus jamais de tour muet. Backend + frontend, tests verts.

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

crates/app-tauri/src/dto.rs

@@ -1750,10 +1750,11 @@ 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 {
@@ -1775,6 +1776,12 @@ 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