test(cli): cover _prepend_note_to_message str/list handling

Regression coverage for the multimodal-message TypeError: note folding into
text parts, image-only insertion, empty-note passthrough, and unknown-shape
fail-open.

tests/cli/test_prepend_note_to_message.py

@@ -0,0 +1,60 @@
+"""Tests for cli._prepend_note_to_message.
+
+Regression coverage for the TypeError raised when a queued /model or
+/reload-skills note was prepended to a multimodal (image-attached) message:
+``can only concatenate str (not "list") to str``.
+"""
+
+from cli import _prepend_note_to_message
+
+
+def test_string_message_gets_note_prepended():
+    assert _prepend_note_to_message("hello", "NOTE") == "NOTE\n\nhello"
+
+
+def test_empty_note_returns_message_unchanged():
+    assert _prepend_note_to_message("hello", "") == "hello"
+    parts = [{"type": "text", "text": "hi"}]
+    assert _prepend_note_to_message(parts, "") == parts
+
+
+def test_list_message_folds_note_into_first_text_part():
+    message = [
+        {"type": "text", "text": "describe this"},
+        {"type": "image_url", "image_url": {"url": "data:..."}},
+    ]
+    result = _prepend_note_to_message(message, "NOTE")
+
+    assert result[0]["type"] == "text"
+    assert result[0]["text"] == "NOTE\n\ndescribe this"
+    # Image part is preserved untouched.
+    assert result[1] == {"type": "image_url", "image_url": {"url": "data:..."}}
+    # Original message is not mutated.
+    assert message[0]["text"] == "describe this"
+
+
+def test_image_only_list_gets_leading_text_part():
+    message = [{"type": "image_url", "image_url": {"url": "data:..."}}]
+    result = _prepend_note_to_message(message, "NOTE")
+
+    assert result[0] == {"type": "text", "text": "NOTE"}
+    assert result[1]["type"] == "image_url"
+
+
+def test_list_message_does_not_raise_typeerror():
+    # The exact #repro shape: multimodal list + queued note must not raise
+    # "can only concatenate str (not 'list') to str".
+    message = [
+        {"type": "text", "text": "look"},
+        {"type": "image_url", "image_url": {"url": "x"}},
+    ]
+    result = _prepend_note_to_message(
+        message, "Model switched to gpt-5.5 (provider: openai-codex)."
+    )
+    assert isinstance(result, list)
+    assert result[0]["text"].startswith("Model switched to gpt-5.5")
+
+
+def test_unknown_shape_returned_unchanged():
+    assert _prepend_note_to_message(123, "NOTE") == 123
+    assert _prepend_note_to_message(None, "NOTE") is None