feat: Copy / Insert / Replace buttons on assistant messages

Each assistant message in the inline prompt history now renders a
small toolbar with three action buttons:

  📋 复制  — copy the rendered assistant content (the markdown
             source, not the rendered HTML) to the clipboard via
             navigator.clipboard.writeText, with a fallback to a
             hidden textarea + execCommand for non-secure contexts.
  ↪ 插入  — insert the content at the current editor cursor. Uses
             cell.editor.getCursorPosition() + the cell source to
             compute the offset, then sharedModel.setSource to splice
             the text in. Falls back to appending if the cursor can't
             be read.
  ⟳ 替换  — replace the entire cell source with the assistant content
             via sharedModel.setSource.

User messages and the textarea/send/cancel UI are unchanged; the
toolbar only appears on rendered assistant (markdown) messages.

Implementation:
  - OpenCodeInlinePrompt now stores the cell (private _cell) so the
    button handlers can act on it.
  - New private methods: _copyToClipboard, _insertAtCursor,
    _replaceCell. Each shows a Notification on success/failure.
  - Added import { Notification } from '@jupyterlab/apputils' (the
    prompt file didn't import it before; cell_actions does, so the
    test mock already provides the static methods).
  - style/base.css: .opencode-msg-toolbar (flex row) and
    .opencode-msg-btn (small bordered button) styles.
  - Test fake cell now has an editor stub returning cursor (0,0) so
    the Insert test produces a deterministic offset. jsdom lacks
    navigator.clipboard; the test stubs it globally.

Tests:
  - jest 33/33 (was 29; +4 new: toolbar renders 3 buttons only on
    assistant, Replace sets source, Insert splices at offset 0, Copy
    calls clipboard.writeText with the content).
  - pytest 37/37 (no server changes).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/__tests__/opencode_cell_actions.spec.ts

@@ -52,6 +52,15 @@ jest.mock('marked', () => ({
   marked: { parse: (md: string) => `<mock>${md}</mock>` }
 }));
 
+// jsdom doesn't implement navigator.clipboard; stub it so the copy
+// button's happy path can be exercised in tests.
+beforeAll(() => {
+  Object.defineProperty(globalThis.navigator, 'clipboard', {
+    value: { writeText: jest.fn().mockResolvedValue(undefined) },
+    configurable: true
+  });
+});
+
 // Mock the network-touching API module so jsdom tests don't try to hit
 // a real notebook server (and to keep the refresh-history logic decoupled
 // from the network). The cell_actions tests focus on widget behavior.
@@ -101,6 +110,11 @@ function makeFakeCell(
   const cell = new (CodeCell as unknown as new () => CodeCell)();
   (cell as any).model = model;
   (cell as any).node = document.createElement('div');
+  // Minimal editor stub: cursor always at (line 0, column 0) so the
+  // "insert at cursor" tests produce a deterministic offset.
+  (cell as any).editor = {
+    getCursorPosition: () => ({ line: 0, column: 0 })
+  };
 
   const notebook: any = { widgets: [] as CodeCell[] };
   const panel: any = {
@@ -417,4 +431,97 @@ describe('OpenCodeInlinePrompt', () => {
     expect(asstMsg.innerHTML).toContain('<mock>');
     expect(asstMsg.innerHTML).toContain('# Here you go');
   });
+
+  it('renders a Copy / Insert / Replace toolbar on assistant messages', () => {
+    const cell = makeFakeCell('x = 1');
+    const prompt = new OpenCodeInlinePrompt(cell, {
+      onSubmit: jest.fn(),
+      onCancel: jest.fn(),
+      disabled: false,
+      providers: null
+    });
+    prompt.setMessages([
+      { role: 'user', content: 'help' },
+      { role: 'assistant', content: '```py\nprint(1)\n```' }
+    ]);
+
+    const asst = prompt.node.querySelector(
+      '.opencode-msg-assistant'
+    ) as HTMLElement;
+    const toolbar = asst.querySelector(
+      '.opencode-msg-toolbar'
+    ) as HTMLElement;
+    expect(toolbar).not.toBeNull();
+    const buttons = toolbar.querySelectorAll('button');
+    expect(buttons.length).toBe(3);
+    expect(buttons[0].textContent).toContain('复制');
+    expect(buttons[1].textContent).toContain('插入');
+    expect(buttons[2].textContent).toContain('替换');
+    // User messages must NOT get a toolbar.
+    const user = prompt.node.querySelector('.opencode-msg-user');
+    expect(user!.querySelector('.opencode-msg-toolbar')).toBeNull();
+  });
+
+  it('Replace button overwrites the cell source with the assistant content', () => {
+    const cell = makeFakeCell('x = 1');
+    const prompt = new OpenCodeInlinePrompt(cell, {
+      onSubmit: jest.fn(),
+      onCancel: jest.fn(),
+      disabled: false,
+      providers: null
+    });
+    prompt.setMessages([
+      { role: 'assistant', content: 'print("replaced")' }
+    ]);
+    const replaceBtn = prompt.node.querySelector(
+      '.opencode-msg-assistant .opencode-msg-toolbar button:nth-child(3)'
+    ) as HTMLButtonElement;
+    replaceBtn.click();
+
+    const setSource = (cell as any).model.sharedModel.setSource as jest.Mock;
+    expect(setSource).toHaveBeenCalledTimes(1);
+    expect(setSource).toHaveBeenCalledWith('print("replaced")');
+  });
+
+  it('Insert button splices the content at the editor cursor (offset 0)', () => {
+    const cell = makeFakeCell('x = 1');
+    const prompt = new OpenCodeInlinePrompt(cell, {
+      onSubmit: jest.fn(),
+      onCancel: jest.fn(),
+      disabled: false,
+      providers: null
+    });
+    prompt.setMessages([
+      { role: 'assistant', content: 'inserted()\n' }
+    ]);
+    const insertBtn = prompt.node.querySelector(
+      '.opencode-msg-assistant .opencode-msg-toolbar button:nth-child(2)'
+    ) as HTMLButtonElement;
+    insertBtn.click();
+
+    const setSource = (cell as any).model.sharedModel.setSource as jest.Mock;
+    expect(setSource).toHaveBeenCalledTimes(1);
+    // cursor at (0,0) with source "x = 1" -> content is inserted at offset 0
+    expect(setSource).toHaveBeenCalledWith('inserted()\nx = 1');
+  });
+
+  it('Copy button writes the assistant content to the clipboard', () => {
+    const cell = makeFakeCell('x = 1');
+    const prompt = new OpenCodeInlinePrompt(cell, {
+      onSubmit: jest.fn(),
+      onCancel: jest.fn(),
+      disabled: false,
+      providers: null
+    });
+    prompt.setMessages([
+      { role: 'assistant', content: 'copy me' }
+    ]);
+    const copyBtn = prompt.node.querySelector(
+      '.opencode-msg-assistant .opencode-msg-toolbar button:nth-child(1)'
+    ) as HTMLButtonElement;
+    copyBtn.click();
+
+    const writeText = (navigator.clipboard.writeText as unknown as jest.Mock);
+    expect(writeText).toHaveBeenCalledWith('copy me');
+  });
 });