Fix/qa defects df002 df011 (#9)

## Summary

What changed and why?

## Change Type

- [ ] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: thanhnv <thanhnv.ip@gmail.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Reviewed-on: #9

tests/test_ai_file_editor_input.py

@@ -0,0 +1,68 @@
+"""DF-008 — presentation/folder/ai_file_editor_dialog.py::_AutoExpandInput.
+
+The AI-edit instruction box was a fixed-height single-line QLineEdit (read as
+cramped); it is now a QPlainTextEdit that grows with content, submits on
+Enter, and inserts a newline on Shift+Enter — same convention as the Cowork
+composer's input (presentation/chat/chat_input_box.py::_Input)."""
+from __future__ import annotations
+
+import os
+
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
+import pytest
+
+QApplication = pytest.importorskip("PySide6.QtWidgets").QApplication
+from PySide6.QtCore import Qt
+from PySide6.QtGui import QKeyEvent
+from PySide6.QtCore import QEvent
+
+from cowork_local.presentation.folder.ai_file_editor_dialog import _AutoExpandInput
+
+
+@pytest.fixture(scope="module")
+def qapp():
+    app = QApplication.instance() or QApplication([])
+    yield app
+
+
+def _press_enter(widget, shift: bool = False) -> None:
+    mods = Qt.ShiftModifier if shift else Qt.NoModifier
+    event = QKeyEvent(QEvent.KeyPress, Qt.Key_Return, mods)
+    widget.keyPressEvent(event)
+
+
+def test_starts_at_min_height(qapp) -> None:
+    box = _AutoExpandInput()
+    assert box.height() == _AutoExpandInput.MIN_HEIGHT
+
+
+def test_grows_with_multiline_content(qapp) -> None:
+    box = _AutoExpandInput()
+    start_height = box.height()
+    box.setPlainText("\n".join(f"line {i}" for i in range(10)))
+    assert box.height() > start_height
+    assert box.height() <= _AutoExpandInput.MAX_HEIGHT
+
+
+def test_enter_emits_submit_and_does_not_insert_newline(qapp) -> None:
+    box = _AutoExpandInput()
+    box.setPlainText("hello")
+    received = []
+    box.submit.connect(lambda: received.append(True))
+    _press_enter(box)
+    assert received == [True]
+    assert box.toPlainText() == "hello"   # Enter did not add a newline
+
+
+def test_shift_enter_inserts_newline_without_submitting(qapp) -> None:
+    box = _AutoExpandInput()
+    box.setPlainText("hello")
+    cursor = box.textCursor()
+    cursor.movePosition(cursor.MoveOperation.End)
+    box.setTextCursor(cursor)
+    received = []
+    box.submit.connect(lambda: received.append(True))
+    _press_enter(box, shift=True)
+    assert received == []
+    assert box.toPlainText() == "hello\n"