fix: merging consecutive user messages corrupts multimodal (image) content (#1277)

* fix: preserve multimodal content blocks when merging consecutive user messages

* test: consecutive user-message merge keeps multimodal image blocks

tests/test_sanitize_multimodal_merge.py

@@ -0,0 +1,28 @@
+"""Regression: merging consecutive user messages must not str() multimodal content."""
+
+from src.llm_core import _sanitize_llm_messages
+
+
+def test_multimodal_user_message_keeps_image_block_when_merged():
+    image_msg = {"role": "user", "content": [
+        {"type": "text", "text": "look at this"},
+        {"type": "image_url", "image_url": {"url": "data:image/png;base64,AAAA"}},
+    ]}
+    tool_result = {"role": "user", "content": "Tool result: 42"}
+    out = _sanitize_llm_messages([image_msg, tool_result])
+
+    # The two consecutive user messages collapse into one...
+    assert len(out) == 1
+    content = out[0]["content"]
+    # ...and the image block survives (it used to be str()-ed into a repr).
+    assert isinstance(content, list)
+    assert any(b.get("type") == "image_url" for b in content)
+    assert content[-1] == {"type": "text", "text": "Tool result: 42"}
+
+
+def test_string_only_user_merge_unchanged():
+    a = {"role": "user", "content": "first"}
+    b = {"role": "user", "content": "second"}
+    out = _sanitize_llm_messages([a, b])
+    assert len(out) == 1
+    assert out[0]["content"] == "first\n\nsecond"