fix(ollama): sanitize null content on every message shape

gemma thinking-only assistant turns carry content: null with no
tool_calls, slipping past the tool-call-only sanitizer and 400ing the
whole agent run ('invalid message content type: <nil>'). Null content is
now blanked for any role.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ollama/provider.py

@@ -113,14 +113,18 @@ def _sanitize_messages(messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
     for msg in messages:
         msg_copy = dict(msg)
 
-        # Fix null content in assistant messages with tool calls
-        if msg_copy.get("role") == "assistant":
-            if msg_copy.get("content") is None and msg_copy.get("tool_calls"):
-                msg_copy["content"] = ""
-                logger.debug(
-                    "sanitized_null_content",
-                    tool_call_count=len(msg_copy["tool_calls"]),
-                )
+        # Fix null content in ANY message: Ollama rejects content: null with
+        # "invalid message content type: <nil>". The tool-call-only assistant
+        # case is the common one, but gemma thinking-only turns produce
+        # assistant messages with null content and NO tool_calls, which
+        # previously slipped through and 400'd the whole agent run.
+        if "content" in msg_copy and msg_copy.get("content") is None:
+            msg_copy["content"] = ""
+            logger.debug(
+                "sanitized_null_content",
+                role=msg_copy.get("role"),
+                tool_call_count=len(msg_copy.get("tool_calls") or []),
+            )
 
         sanitized.append(msg_copy)