fix(agent): drop the empty assistant turn from an approved-action replay (#6124)

The approved-action replay appends the sealed tool result with no assistant
prose for that round, which produced an assistant message with content "".
Anthropic's Messages API rejects a non-final assistant message with empty
content, so a resumed turn after a tool approval failed before the model saw
the result. A turn carrying neither prose nor reasoning has nothing to say to
any provider, so it is no longer appended. A round with prose, and a
reasoning-only round that DeepSeek thinking mode needs, both still append.

src/agent_loop.py

@@ -3081,10 +3081,17 @@ def _append_tool_results(
             messages.append(result_message)
     else:
         tool_output_text = "\n\n".join(tool_results)
-        msg = {"role": "assistant", "content": round_response}
-        if round_reasoning:
-            msg["reasoning_content"] = round_reasoning
-        messages.append(msg)
+        # An approved-action replay injects the sealed tool result with no
+        # assistant prose for that round, which used to append an assistant turn
+        # whose content was "". Anthropic's Messages API rejects a non-final
+        # assistant message with empty content (HTTP 400), so the resumed turn
+        # died before the model saw the result. A turn carrying neither prose nor
+        # reasoning has nothing to say to any provider, so skip it entirely.
+        if round_response.strip() or round_reasoning:
+            msg = {"role": "assistant", "content": round_response}
+            if round_reasoning:
+                msg["reasoning_content"] = round_reasoning
+            messages.append(msg)
         # Tool output (shell/python stdout, file reads, fetched pages, email
         # bodies, MCP results) is sourced from outside the server. Wrap it as
         # untrusted data so prompt-injection inside a tool result is treated as