Surface isError in ToolResultPanel

Show a red alert banner when CallToolResult.isError is true, giving
a visual distinction between successful and failed tool executions.
Add ErrorResult story to demonstrate the error state.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clients/web/src/components/groups/ToolResultPanel/ToolResultPanel.stories.tsx

@@ -89,3 +89,17 @@ export const MixedContent: Story = {
     result: mixedResult,
   },
 };
+
+export const ErrorResult: Story = {
+  args: {
+    result: {
+      isError: true,
+      content: [
+        {
+          type: "text",
+          text: "ENOENT: no such file or directory, open '/data/missing.txt'",
+        },
+      ],
+    },
+  },
+};

clients/web/src/components/groups/ToolResultPanel/ToolResultPanel.tsx

@@ -1,4 +1,4 @@
-import { Button, Group, Stack, Text, Title } from "@mantine/core";
+import { Alert, Button, Group, Stack, Text, Title } from "@mantine/core";
 import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
 import { ContentViewer } from "../../elements/ContentViewer/ContentViewer";
 
@@ -19,6 +19,11 @@ export function ToolResultPanel({ result, onClear }: ToolResultPanelProps) {
         <Title order={4}>Results</Title>
         <ClearButton onClick={onClear}>Clear</ClearButton>
       </Group>
+      {result.isError && (
+        <Alert color="red" variant="light">
+          Tool execution returned an error
+        </Alert>
+      )}
       {result.content.length === 0 ? (
         <Text c="dimmed">No results yet</Text>
       ) : (