test(kilo-ui): add storybook story for edit tool diff viewer button

Introduce `WithEditToolOpenDiffAction` story and supporting
`editCompletedPart` fixture with full filediff metadata to exercise
the `canOpenDiff()` code path. Force-reveal the hover-only action
slot via a scoped style override so visual regression snapshots
capture the icon-button layout.

- Add completed edit tool part with before/after file content
- Wire `onOpenDiff` prop through `AllProviders` into `DataProvider`
- Create dedicated `mockDataEdit` dataset for the new story

packages/kilo-ui/src/stories/message-part.stories.tsx

@@ -163,6 +163,39 @@ const errorToolPart: ToolPart = {
   },
 }
 
+// Completed edit tool with full filediff metadata — exercises canOpenDiff()
+// so the "Open in Diff Viewer" icon button renders in the trigger.
+const editCompletedPart: ToolPart = {
+  id: "part-tool-edit-done",
+  sessionID: SESSION_ID,
+  messageID: ASST_MSG_ID,
+  type: "tool",
+  callID: "call-edit-done",
+  tool: "edit",
+  state: {
+    status: "completed",
+    input: {
+      filePath: "src/counter.tsx",
+      oldString: "const [count, setCount] = createSignal(0)",
+      newString: "const [count, setCount] = createSignal(1)",
+    },
+    output: "File edited successfully",
+    title: "Edit file",
+    metadata: {
+      filediff: {
+        file: "src/counter.tsx",
+        before:
+          "import { createSignal } from 'solid-js'\n\nexport function Counter() {\n  const [count, setCount] = createSignal(0)\n  return <button onClick={() => setCount(count() + 1)}>{count()}</button>\n}\n",
+        after:
+          "import { createSignal } from 'solid-js'\n\nexport function Counter() {\n  const [count, setCount] = createSignal(1)\n  return <button onClick={() => setCount(count() + 1)}>{count()}</button>\n}\n",
+        additions: 1,
+        deletions: 1,
+      },
+    },
+    time: { start: now - 4000, end: now - 3500 },
+  },
+}
+
 // --- Reasoning part ---
 
 const reasoningPart: ReasoningPart = {
@@ -208,10 +241,12 @@ const mockDataReasoning = createMockData([reasoningPart, textPart])
 const mockDataBash = createMockData([bashCompleted])
 // Three context-group tools — exercises ContextToolGroupHeader collapse path
 const mockDataContextGroup = createMockData([completedToolPart, grepCompleted, globCompleted, textPart])
+// Completed edit tool with filediff — exercises the "Open in Diff Viewer" button path
+const mockDataEdit = createMockData([editCompletedPart])
 
-function AllProviders(props: { children: any; data?: MockData }) {
+function AllProviders(props: { children: any; data?: MockData; onOpenDiff?: () => void }) {
   return (
-    <DataProvider data={props.data ?? mockData} directory="/project">
+    <DataProvider data={props.data ?? mockData} directory="/project" onOpenDiff={props.onOpenDiff}>
       <DiffComponentProvider component={Diff}>
         <CodeComponentProvider component={Code}>
           <FileComponentProvider component={File}>
@@ -354,6 +389,25 @@ export const WithContextGroup: Story = {
   ),
 }
 
+// --- Completed edit tool with filediff → "Open in Diff Viewer" icon visible ---
+//
+// CSS :hover cannot be triggered reliably from Storybook `play` functions,
+// so the action slot is force-revealed via a scoped style override. This
+// captures the layout with the icon present so the visual regression
+// suite catches any regression in:
+//   - the `:has()` parent-grow rule on basic-tool-tool-info
+//   - the `margin-left: auto` pushing the action to the right
+//   - the icon-button ghost variant in the tool-trigger row
+export const WithEditToolOpenDiffAction: Story = {
+  name: "WithEditTool (open-diff action visible)",
+  render: () => (
+    <AllProviders data={mockDataEdit} onOpenDiff={() => {}}>
+      <style>{`[data-slot="edit-trigger-actions"] { opacity: 1 !important; }`}</style>
+      <AssistantParts messages={[mockAssistantMessage]} />
+    </AllProviders>
+  ),
+}
+
 // --- All 5 tool hint error types in a single screenshot ---
 
 const hintErrors: ToolPart[] = [