feat: allow multi-line input (#438)

## Description
This PR implements multi-line input support for Codex when it asks for
user feedback (Issue #344). Users can now use Shift+Enter to add new
lines in their responses, making it easier to provide formatted code
snippets, lists, or other structured content.

## Changes
- Replace the single-line TextInput component with the
MultilineTextEditor component in terminal-chat-input.tsx
- Add support for Shift+Enter to create new lines
- Update key handling logic to properly handle history navigation in a
multi-line context
- Add reference to the editor to access cursor position information
- Update help text to inform users about the Shift+Enter functionality
- Add tests for the new functionality

## Testing
- Added new test file (terminal-chat-input-multiline.test.tsx) to test
the multi-line input functionality
- All existing tests continue to pass
- Manually tested the feature to ensure it works as expected

## Fixes
Closes #344

## Screenshots
N/A

## Additional Notes
This implementation maintains backward compatibility while adding the
requested multi-line input functionality. The UI remains clean and
intuitive, with a simple hint about using Shift+Enter for new lines.

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>

codex-cli/tests/terminal-chat-input-multiline.test.tsx

@@ -0,0 +1,157 @@
+import React from "react";
+import type { ComponentProps } from "react";
+import { renderTui } from "./ui-test-helpers.js";
+import TerminalChatInput from "../src/components/chat/terminal-chat-input.js";
+import { describe, it, expect, vi } from "vitest";
+
+// Helper that lets us type and then immediately flush ink's async timers
+async function type(
+  stdin: NodeJS.WritableStream,
+  text: string,
+  flush: () => Promise<void>,
+) {
+  stdin.write(text);
+  await flush();
+}
+
+// Mock the createInputItem function to avoid filesystem operations
+vi.mock("../src/utils/input-utils.js", () => ({
+  createInputItem: vi.fn(async (text: string) => ({
+    role: "user",
+    type: "message",
+    content: [{ type: "input_text", text }],
+  })),
+}));
+
+describe("TerminalChatInput multiline functionality", () => {
+  it("renders the multiline editor component", async () => {
+    const props: ComponentProps<typeof TerminalChatInput> = {
+      isNew: false,
+      loading: false,
+      submitInput: () => {},
+      confirmationPrompt: null,
+      explanation: undefined,
+      submitConfirmation: () => {},
+      setLastResponseId: () => {},
+      setItems: () => {},
+      contextLeftPercent: 50,
+      openOverlay: () => {},
+      openDiffOverlay: () => {},
+      openModelOverlay: () => {},
+      openApprovalOverlay: () => {},
+      openHelpOverlay: () => {},
+      onCompact: () => {},
+      interruptAgent: () => {},
+      active: true,
+      thinkingSeconds: 0,
+    };
+
+    const { lastFrameStripped } = renderTui(<TerminalChatInput {...props} />);
+    const frame = lastFrameStripped();
+
+    // Check that the help text mentions shift+enter for new line
+    expect(frame).toContain("shift+enter for new line");
+  });
+
+  it("allows multiline input with shift+enter", async () => {
+    const submitInput = vi.fn();
+
+    const props: ComponentProps<typeof TerminalChatInput> = {
+      isNew: false,
+      loading: false,
+      submitInput,
+      confirmationPrompt: null,
+      explanation: undefined,
+      submitConfirmation: () => {},
+      setLastResponseId: () => {},
+      setItems: () => {},
+      contextLeftPercent: 50,
+      openOverlay: () => {},
+      openDiffOverlay: () => {},
+      openModelOverlay: () => {},
+      openApprovalOverlay: () => {},
+      openHelpOverlay: () => {},
+      onCompact: () => {},
+      interruptAgent: () => {},
+      active: true,
+      thinkingSeconds: 0,
+    };
+
+    const { stdin, lastFrameStripped, flush, cleanup } = renderTui(
+      <TerminalChatInput {...props} />,
+    );
+
+    // Type some text
+    await type(stdin, "first line", flush);
+
+    // Send Shift+Enter (CSI-u format)
+    await type(stdin, "\u001B[13;2u", flush);
+
+    // Type more text
+    await type(stdin, "second line", flush);
+
+    // Check that both lines are visible in the editor
+    const frame = lastFrameStripped();
+    expect(frame).toContain("first line");
+    expect(frame).toContain("second line");
+
+    // Submit the multiline input with Enter
+    await type(stdin, "\r", flush);
+
+    // Check that submitInput was called with the multiline text
+    expect(submitInput).toHaveBeenCalledTimes(1);
+
+    cleanup();
+  });
+
+  it("allows multiline input with shift+enter (modifyOtherKeys=1 format)", async () => {
+    const submitInput = vi.fn();
+
+    const props: ComponentProps<typeof TerminalChatInput> = {
+      isNew: false,
+      loading: false,
+      submitInput,
+      confirmationPrompt: null,
+      explanation: undefined,
+      submitConfirmation: () => {},
+      setLastResponseId: () => {},
+      setItems: () => {},
+      contextLeftPercent: 50,
+      openOverlay: () => {},
+      openDiffOverlay: () => {},
+      openModelOverlay: () => {},
+      openApprovalOverlay: () => {},
+      openHelpOverlay: () => {},
+      onCompact: () => {},
+      interruptAgent: () => {},
+      active: true,
+      thinkingSeconds: 0,
+    };
+
+    const { stdin, lastFrameStripped, flush, cleanup } = renderTui(
+      <TerminalChatInput {...props} />,
+    );
+
+    // Type some text
+    await type(stdin, "first line", flush);
+
+    // Send Shift+Enter (modifyOtherKeys=1 format)
+    await type(stdin, "\u001B[27;2;13~", flush);
+
+    // Type more text
+    await type(stdin, "second line", flush);
+
+    // Check that both lines are visible in the editor
+    const frame = lastFrameStripped();
+    expect(frame).toContain("first line");
+    expect(frame).toContain("second line");
+
+    // Submit the multiline input with Enter
+    await type(stdin, "\r", flush);
+
+    // Check that submitInput was called with the multiline text
+    expect(submitInput).toHaveBeenCalledTimes(1);
+
+    cleanup();
+  });
+});