fix(deepseek): more aggressive R1 fix and detailed error logging

- Ensure assistant tool calls always have content: "" instead of null.
- Add debug-level logging of the full request body when DeepSeek returns 400.
- Improved R1 placeholder injection to include content sanitation.

src/providers/helpers.rs

@@ -65,10 +65,10 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
             msg["reasoning_content"] = serde_json::json!(reasoning);
         }
 
-        // For assistant messages with tool_calls, content can be null
+        // For assistant messages with tool_calls, content can be empty string
         if let Some(tool_calls) = &m.tool_calls {
             if parts.is_empty() {
-                msg["content"] = serde_json::Value::Null;
+                msg["content"] = serde_json::json!("");
             } else {
                 msg["content"] = serde_json::json!(parts);
             }
@@ -143,10 +143,10 @@ pub async fn messages_to_openai_json_text_only(
             msg["reasoning_content"] = serde_json::json!(reasoning);
         }
 
-        // For assistant messages with tool_calls, content can be null
+        // For assistant messages with tool_calls, content can be empty string
         if let Some(tool_calls) = &m.tool_calls {
             if parts.is_empty() {
-                msg["content"] = serde_json::Value::Null;
+                msg["content"] = serde_json::json!("");
             } else {
                 msg["content"] = serde_json::json!(parts);
             }