feat(chat): valide le routing cellKind=chat côté frontend et corrige UX

- Ajoute cellKind dans AgentChatHandle et vérifie que launchAgentChat reçoit cellKind:chat
- Rejette avec erreur STRUCTURED_ROUTED_TO_PTY si le backend route vers PTY
- Ajoute cellKind:chat dans MockAgentGateway.launchAgentChat
- Corrige l'overflow du layout CustomAgentChatView (bounded shell avec scroll + composer fixe)
- Dédouble les prompts utilisateur quand le stream les echo
- Ajoute les tests launchAgentChat returns handle only when confirms chat et launchAgentChat rejects PTY-routed launch
- Ajoute les tests does not duplicate user prompt when echoed et keeps chat shell bounded

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

frontend/src/features/agents/CustomAgentChatView.tsx

@@ -93,6 +93,12 @@ function appendAgentDelta(turns: ChatTurn[], text: string): ChatTurn[] {
   return next;
 }
 
+function appendUserPrompt(turns: ChatTurn[], text: string): ChatTurn[] {
+  const last = turns[turns.length - 1];
+  if (last?.role === "user" && last.text === text) return turns;
+  return [...turns, { role: "user", text }];
+}
+
 function foldChunk(turns: ChatTurn[], raw: unknown): ChatTurn[] {
   if (!isReplyRecord(raw)) {
     return [...turns, { role: "unknown", text: unknownChunkLabel(raw) }];
@@ -119,7 +125,7 @@ function foldChunk(turns: ChatTurn[], raw: unknown): ChatTurn[] {
     }
     case "userPrompt":
     case "UserPrompt":
-      return [...turns, { role: "user", text: String(raw.text ?? raw.prompt ?? "") }];
+      return appendUserPrompt(turns, String(raw.text ?? raw.prompt ?? ""));
     default:
       return [...turns, { role: "unknown", text: unknownChunkLabel(raw) }];
   }
@@ -481,20 +487,31 @@ export function CustomAgentChatView({
   return (
     <div
       data-testid="custom-agent-chat-view"
-      className="flex h-full min-h-0 flex-col bg-surface text-content"
+      className="flex h-full min-h-0 flex-col overflow-hidden bg-surface text-content"
     >
-      <div className="flex shrink-0 items-center justify-between gap-2 border-b border-border px-3 py-2">
-        <div className="min-w-0">
+      <div
+        role="toolbar"
+        aria-label="custom agent chat actions"
+        className="flex shrink-0 items-center gap-2 overflow-hidden border-b border-border px-3 py-2"
+      >
+        <div className="min-w-0 flex-1 overflow-hidden">
           <div className="truncate text-sm font-medium">{agentName}</div>
           <div className="truncate text-xs text-muted">
             CLI custom · {profile.name}
           </div>
         </div>
-        {(opening || busy) && (
-          <Button size="sm" variant="danger" onClick={() => void cancel()}>
-            Cancel
-          </Button>
-        )}
+        <div className="flex shrink-0 items-center justify-end gap-2">
+          {(opening || busy) && (
+            <Button
+              size="sm"
+              variant="danger"
+              className="shrink-0 whitespace-nowrap"
+              onClick={() => void cancel()}
+            >
+              Cancel
+            </Button>
+          )}
+        </div>
       </div>
 
       {!supported && (
@@ -508,7 +525,11 @@ export function CustomAgentChatView({
         </p>
       )}
 
-      <div ref={scrollRef} className="flex min-h-0 flex-1 flex-col gap-2 overflow-auto px-3 py-3">
+      <div
+        ref={scrollRef}
+        data-testid="custom-agent-chat-scroll"
+        className="flex min-h-0 flex-1 basis-0 flex-col gap-2 overflow-x-hidden overflow-y-auto px-3 py-3"
+      >
         {opening && turns.length === 0 ? (
           <div className="flex items-center gap-2 text-sm text-muted">
             <Spinner size={14} />
@@ -523,7 +544,10 @@ export function CustomAgentChatView({
         )}
       </div>
 
-      <div className="flex shrink-0 flex-col gap-2 border-t border-border bg-raised/40 p-2">
+      <div
+        data-testid="custom-agent-chat-composer"
+        className="flex shrink-0 flex-col gap-2 border-t border-border bg-raised/40 p-2"
+      >
         {attachment && (
           <div className="flex items-center justify-between gap-2 rounded-md border border-border bg-surface px-2 py-1 text-xs text-muted">
             <span className="truncate">Fichier joint: {attachment}</span>
@@ -532,7 +556,7 @@ export function CustomAgentChatView({
             </button>
           </div>
         )}
-        <div className="flex items-end gap-2">
+        <div className="flex min-w-0 items-end gap-2">
           <textarea
             aria-label={`message CLI custom ${nodeId}`}
             className={cn(
@@ -551,10 +575,22 @@ export function CustomAgentChatView({
               }
             }}
           />
-          <Button size="sm" variant="ghost" disabled={!supported || opening || busy} onClick={() => void pickAttachment()}>
+          <Button
+            size="sm"
+            variant="ghost"
+            className="shrink-0 whitespace-nowrap"
+            disabled={!supported || opening || busy}
+            onClick={() => void pickAttachment()}
+          >
             Joindre
           </Button>
-          <Button size="sm" disabled={!canSend} loading={busy} onClick={() => void send()}>
+          <Button
+            size="sm"
+            className="shrink-0 whitespace-nowrap"
+            disabled={!canSend}
+            loading={busy}
+            onClick={() => void send()}
+          >
             Envoyer
           </Button>
         </div>
@@ -566,8 +602,8 @@ export function CustomAgentChatView({
 function ChatBubble({ turn }: { turn: ChatTurn }) {
   if (turn.role === "tool") {
     return (
-      <div className="flex justify-center">
-        <span className="max-w-[80%] rounded-md border border-border bg-raised px-2 py-1 text-xs text-muted">
+      <div className="flex min-w-0 justify-center">
+        <span className="max-w-[80%] truncate rounded-md border border-border bg-raised px-2 py-1 text-xs text-muted">
           {turn.label}
         </span>
       </div>
@@ -575,8 +611,8 @@ function ChatBubble({ turn }: { turn: ChatTurn }) {
   }
   if (turn.role === "final") {
     return (
-      <div className="flex justify-end">
-        <div className="max-w-[86%] rounded-md border border-success/50 bg-success/10 px-3 py-2 text-sm text-content">
+      <div className="flex min-w-0 justify-end">
+        <div className="min-w-0 max-w-[86%] overflow-hidden rounded-md border border-success/50 bg-success/10 px-3 py-2 text-sm text-content">
           <div className="mb-1 text-xs font-semibold text-success">Task Complete</div>
           <p className="whitespace-pre-wrap break-words">{turn.text}</p>
         </div>
@@ -585,8 +621,8 @@ function ChatBubble({ turn }: { turn: ChatTurn }) {
   }
   if (turn.role === "error" || turn.role === "unknown") {
     return (
-      <div className="flex justify-center">
-        <div className="max-w-[86%] rounded-md border border-danger/40 bg-danger/10 px-3 py-2 text-xs text-danger">
+      <div className="flex min-w-0 justify-center">
+        <div className="min-w-0 max-w-[86%] overflow-hidden rounded-md border border-danger/40 bg-danger/10 px-3 py-2 text-xs text-danger">
           {turn.role === "unknown" ? "Chunk inconnu reçu: " : ""}
           <span className="whitespace-pre-wrap break-words">{turn.text}</span>
         </div>
@@ -596,10 +632,10 @@ function ChatBubble({ turn }: { turn: ChatTurn }) {
 
   const user = turn.role === "user";
   return (
-    <div className={cn("flex", user ? "justify-start" : "justify-end")}>
+    <div className={cn("flex min-w-0", user ? "justify-start" : "justify-end")}>
       <div
         className={cn(
-          "max-w-[82%] rounded-lg border px-3 py-2 text-sm text-content",
+          "min-w-0 max-w-[82%] overflow-hidden rounded-lg border px-3 py-2 text-sm text-content",
           user ? "border-border bg-raised" : "border-primary/25 bg-primary/10",
         )}
       >