refactor(cli): normalize note and avoid blank lines in prepend helper

Adopt the cleaner handling from PR #37080: coerce/strip the note and
skip the extra newlines when the underlying message (or text part) is
empty, while keeping the safer fail-open behavior for unknown shapes.

cli.py

@@ -2127,21 +2127,23 @@ def _prepend_note_to_message(message, note: str):
     sending a pasted image in the same turn.
 
     Returns the message with ``note`` prepended:
-      * ``str``  → ``f"{note}\\n\\n{message}"``
+      * ``str``  → ``f"{note}\\n\\n{message}"`` (just ``note`` when empty)
       * ``list`` → note folded into the first text part, or inserted as a new
         leading ``{"type": "text"}`` part when there is no text part.
     Unknown shapes are returned unchanged (fail-open).
     """
+    note = str(note or "").strip()
     if not note:
         return message
     if isinstance(message, str):
-        return f"{note}\n\n{message}"
+        return f"{note}\n\n{message}" if message else note
     if isinstance(message, list):
         parts = list(message)
         for i, part in enumerate(parts):
             if isinstance(part, dict) and part.get("type") == "text":
                 merged = dict(part)
-                merged["text"] = f"{note}\n\n{part.get('text', '')}"
+                text = merged.get("text", "")
+                merged["text"] = f"{note}\n\n{text}" if text else note
                 parts[i] = merged
                 return parts
         # No text part (image-only) — insert the note as a leading text block.

tests/cli/test_prepend_note_to_message.py

@@ -14,10 +14,30 @@ def test_string_message_gets_note_prepended():
 
 def test_empty_note_returns_message_unchanged():
     assert _prepend_note_to_message("hello", "") == "hello"
+    assert _prepend_note_to_message("hello", "   ") == "hello"
     parts = [{"type": "text", "text": "hi"}]
     assert _prepend_note_to_message(parts, "") == parts
 
 
+def test_note_is_stripped():
+    assert _prepend_note_to_message("hello", "  NOTE  ") == "NOTE\n\nhello"
+
+
+def test_empty_string_message_yields_just_note():
+    # No trailing blank lines when the user message is empty.
+    assert _prepend_note_to_message("", "NOTE") == "NOTE"
+
+
+def test_empty_text_part_yields_just_note():
+    message = [
+        {"type": "text", "text": ""},
+        {"type": "image_url", "image_url": {"url": "x"}},
+    ]
+    result = _prepend_note_to_message(message, "NOTE")
+    assert result[0]["text"] == "NOTE"
+    assert result[1]["type"] == "image_url"
+
+
 def test_list_message_folds_note_into_first_text_part():
     message = [
         {"type": "text", "text": "describe this"},