Keep no-prose assistant tool-call messages through _sanitize_llm_messages (#862)

cb13d09 made _append_tool_results emit content=None (JSON null) for a follow-up
assistant message that carries only tool_calls and no prose, because Gemini's
OpenAI-compatible endpoint and Ollama reject tool_calls alongside an
empty-string content with HTTP 400.

But _sanitize_llm_messages strips None values and then required "content" on
every message, so it dropped that assistant message entirely — leaving the
role:"tool" result dangling with no parent tool_calls, which breaks the
follow-up round for every provider (and regresses ones that accepted "" before,
since the message is now removed rather than sent). cb13d09's tests covered
_append_tool_results in isolation, so the sanitizer interaction was uncaught.

Make the sanitizer role-aware: assistant messages survive with content OR
tool_calls, and a tool-calls-only assistant message gets an explicit
content=None re-added so the provider receives spec-correct `content: null`.
tool messages still require content + tool_call_id; user/system still require
content.

Adds tests/test_llm_core_sanitize_tool_calls.py, which drives the real producer
(_append_tool_results) into the sanitizer and asserts the assistant tool-call
message survives with its tool result paired. Red before this change, green
after.

tests/test_llm_core_sanitize_tool_calls.py

@@ -0,0 +1,61 @@
+"""Regression test: _sanitize_llm_messages must not drop the no-prose
+assistant tool-call message.
+
+Commit cb13d09 changed _append_tool_results so that when the model emits ONLY
+tool calls (no prose), the follow-up assistant message carries content=None
+(JSON null) instead of "" — Google Gemini's OpenAI-compatible endpoint and
+Ollama reject tool_calls alongside an empty-string content with HTTP 400.
+
+But _sanitize_llm_messages drops None values (`v is not None`) and then required
+"content" to be present, so it dropped that assistant message entirely — leaving
+a dangling role:"tool" result with no parent tool_calls. That re-breaks native
+tool-calling on the follow-up round (and regresses providers that accepted ""
+before, since the message is now removed instead of sent). cb13d09's tests only
+exercised _append_tool_results in isolation, so the sanitizer interaction went
+uncaught.
+
+This test drives the real producer (_append_tool_results) into the sanitizer.
+"""
+import sys
+from unittest.mock import MagicMock
+
+# Mock heavy dependencies before importing (mirrors tests/test_agent_loop.py).
+for mod in [
+    'sqlalchemy', 'sqlalchemy.orm', 'sqlalchemy.ext', 'sqlalchemy.ext.declarative',
+    'sqlalchemy.ext.hybrid', 'sqlalchemy.sql', 'sqlalchemy.sql.expression',
+    'src.database', 'src.agent_tools', 'core.models', 'core.database',
+]:
+    if mod not in sys.modules:
+        sys.modules[mod] = MagicMock()
+
+from src.agent_loop import _append_tool_results
+from src.llm_core import _sanitize_llm_messages
+
+
+def test_sanitize_keeps_no_prose_assistant_tool_call_message():
+    native = [{"id": "call_1", "name": "web_fetch",
+               "arguments": '{"url": "https://example.com"}'}]
+    messages = []
+    # Model emitted only a tool call, no prose -> _append_tool_results sets the
+    # assistant message's content to None (cb13d09).
+    _append_tool_results(messages, "", native, [{}], ["page text"],
+                         used_native=True, round_num=1)
+    assert messages[0]["role"] == "assistant"
+    assert messages[0]["content"] is None  # producer contract (cb13d09)
+
+    out = _sanitize_llm_messages(messages)
+    roles = [m["role"] for m in out]
+
+    # The assistant tool-call message must survive sanitization, otherwise the
+    # following tool result is dangling and the provider call breaks.
+    assert "assistant" in roles, (
+        "sanitize dropped the no-prose assistant tool-call message; the tool "
+        "result is left dangling"
+    )
+    assistant = next(m for m in out if m["role"] == "assistant")
+    assert assistant.get("tool_calls"), "assistant tool_calls were lost"
+    # Faithful to cb13d09: keep explicit JSON null rather than an omitted key.
+    assert assistant["content"] is None
+    # Pairing intact: the tool result references the assistant's tool_call id.
+    tool = next(m for m in out if m["role"] == "tool")
+    assert tool["tool_call_id"] == assistant["tool_calls"][0]["id"]