fix(site): optimistic message on agent chat submit (#22422)

## Problem

When sending a message in the agent detail chat, the text lingered in
the input textarea while the HTTP POST round-tripped to the server. Only
after the server responded did the input clear and the message appear in
the timeline (via WebSocket). This created a noticeable delay where the
user couldn't start typing their next message.

## Solution

**Optimistic input clear** (`AgentChatInput.tsx`):
- Clear the textarea and editing state *immediately* on submit, before
awaiting the network call.
- Capture the input text beforehand so it can be restored in the `catch`
block if the request fails.

**Optimistic user bubble** (`AgentDetail.tsx`):
- Inject a temporary `ChatMessage` (with a negative ID) into the chat
store so the user's message bubble appears in the timeline instantly.
- Set chat status to `pending` and clear stream state, mirroring the
existing edit-message path.
- On error, roll back: remove the optimistic message and restore the
previous chat status.

The real message arrives via the WebSocket stream and
`upsertDurableMessage` replaces the optimistic entry naturally (the
server message has a positive ID, so it's inserted alongside; the
optimistic negative-ID message gets cleaned up when `replaceMessages` is
called with the authoritative message list from the next query
invalidation).

## Testing

- Type a message and press Enter — input clears and bubble appears
immediately.
- Simulate a network error — input text is restored, optimistic bubble
is removed.
- Edit an existing message — unchanged behavior (already had optimistic
updates).
- Queue a message while streaming — unchanged behavior.

site/src/pages/AgentsPage/AgentChatInput.tsx

@@ -324,25 +324,35 @@ export const AgentChatInput = memo<AgentChatInputProps>(
 				isEditingHistoryMessage && editingHistoryMessageID !== null
 					? editingHistoryMessageID
 					: undefined;
+			// Capture the raw input before clearing so we can restore
+			// it if the request fails.
+			const capturedInput = input;
+
+			// Clear the input and editing state immediately so the
+			// user can start typing their next message without waiting
+			// for the network round-trip.
+			setInput("");
+			onInputChange?.("");
+			if (queueEditID !== null) {
+				setEditingQueuedMessageID(null);
+				setDraftBeforeQueueEdit(null);
+			}
+			if (isEditingHistoryMessage) {
+				setIsEditingHistoryMessage(false);
+				setEditingHistoryMessageID(null);
+				setDraftBeforeHistoryEdit(null);
+				onEditCleared?.();
+			}
+
 			try {
-				await onSend(input, editedMessageID);
+				await onSend(capturedInput, editedMessageID);
 				if (queueEditID !== null && onDeleteQueuedMessage) {
 					await onDeleteQueuedMessage(queueEditID);
 				}
-				setInput("");
-				onInputChange?.("");
-				if (queueEditID !== null) {
-					setEditingQueuedMessageID(null);
-					setDraftBeforeQueueEdit(null);
-				}
-				if (isEditingHistoryMessage) {
-					setIsEditingHistoryMessage(false);
-					setEditingHistoryMessageID(null);
-					setDraftBeforeHistoryEdit(null);
-					onEditCleared?.();
-				}
 			} catch {
-				// Keep input on failure so the user can retry.
+				// Restore the input so the user can retry.
+				setInput(capturedInput);
+				onInputChange?.(capturedInput);
 			} finally {
 				// Re-focus the textarea so the user can keep typing.
 				textareaRef.current?.focus();

site/src/pages/AgentsPage/AgentDetail.tsx

@@ -614,7 +614,31 @@ const AgentDetail: FC = () => {
 		if (scrollContainerRef.current) {
 			scrollContainerRef.current.scrollTop = 0;
 		}
-		await sendMutation.mutateAsync(request);
+
+		// Inject an optimistic user message so the bubble appears in
+		// the timeline immediately, without waiting for the server.
+		const previousMessages = getOrderedMessagesFromStore(store);
+		const previousChatStatus = store.getSnapshot().chatStatus;
+		const optimisticMessage: TypesGen.ChatMessage = {
+			id: -Date.now(),
+			chat_id: agentId,
+			created_at: new Date().toISOString(),
+			role: "user",
+			content: toOptimisticMessageParts(content),
+		};
+		store.upsertDurableMessage(optimisticMessage);
+		store.clearStreamState();
+		store.setChatStatus("pending");
+
+		try {
+			await sendMutation.mutateAsync(request);
+		} catch (error) {
+			// Roll back the optimistic message so the timeline
+			// returns to its previous state.
+			store.replaceMessages(previousMessages);
+			store.setChatStatus(previousChatStatus);
+			throw error;
+		}
 		if (typeof window !== "undefined") {
 			if (selectedModelConfigID) {
 				localStorage.setItem(