fix(chat): normalize provider-safe message history

surfsense_backend/tests/unit/agents/chat/runtime/test_llm_config_sanitizer.py

@@ -0,0 +1,40 @@
+"""Regression tests for model-boundary message sanitization."""
+
+from __future__ import annotations
+
+import pytest
+from langchain_core.messages import AIMessage
+
+from app.agents.chat.runtime.llm_config import _sanitize_messages
+
+pytestmark = pytest.mark.unit
+
+
+def test_sanitize_messages_strips_provider_specific_thinking_blocks() -> None:
+    original = AIMessage(
+        content=[
+            {"type": "thinking", "thinking": "private reasoning"},
+            {"type": "text", "text": "visible answer"},
+        ]
+    )
+
+    sanitized = _sanitize_messages([original])
+
+    assert sanitized[0].content == "visible answer"
+    assert original.content == [
+        {"type": "thinking", "thinking": "private reasoning"},
+        {"type": "text", "text": "visible answer"},
+    ]
+
+
+def test_sanitize_messages_sets_tool_only_ai_content_to_none() -> None:
+    message = AIMessage(
+        content="",
+        tool_calls=[{"name": "search", "args": {"q": "x"}, "id": "call_1"}],
+    )
+
+    sanitized = _sanitize_messages([message])
+
+    assert sanitized[0].content is None
+    assert message.content == ""
+