fix: DeepSeek/Kimi thinking mode requires reasoning_content on ALL assistant messages

Previously _copy_reasoning_content_for_api only padded reasoning_content
when the assistant message had tool_calls. DeepSeek V4 thinking mode
requires the field on every assistant turn, including plain text replies
without tool_calls.

- Remove the 'source_msg.get("tool_calls") and' guard
- Update test: plain assistant turns now get padded for DeepSeek/Kimi

Fixes #15213

tests/run_agent/test_deepseek_reasoning_content_echo.py

@@ -88,13 +88,13 @@ class TestCopyReasoningContentForApi:
         agent._copy_reasoning_content_for_api(source, api_msg)
         assert api_msg.get("reasoning_content") == ""
 
-    def test_deepseek_assistant_no_tool_call_left_alone(self) -> None:
-        """Plain assistant turns without tool_calls don't get padded."""
+    def test_deepseek_assistant_no_tool_call_gets_padded(self) -> None:
+        """DeepSeek thinking mode pads ALL assistant turns, even without tool_calls."""
         agent = _make_agent(provider="deepseek", model="deepseek-v4-flash")
         source = {"role": "assistant", "content": "hello"}
         api_msg: dict = {}
         agent._copy_reasoning_content_for_api(source, api_msg)
-        assert "reasoning_content" not in api_msg
+        assert api_msg.get("reasoning_content") == ""
 
     def test_deepseek_explicit_reasoning_content_preserved(self) -> None:
         """When reasoning_content is already set, it's copied verbatim."""