fix: revert shared helper changes that broke standard OpenAI models

- Move DeepSeek R1 reasoning_content and content:"" fixes to DeepSeekProvider only.
- Restore OpenAI-standard null content for tool calls in helpers.rs.
- Fixes 400 Bad Request for gpt-5-nano and other strict OpenAI models.

src/providers/helpers.rs

@@ -60,15 +60,10 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
 
         let mut msg = serde_json::json!({ "role": m.role });
 
-        // Include reasoning_content if present (DeepSeek R1/reasoner requires this in history)
-        if let Some(reasoning) = &m.reasoning_content {
-            msg["reasoning_content"] = serde_json::json!(reasoning);
-        }
-
-        // For assistant messages with tool_calls, content can be empty string
+        // For assistant messages with tool_calls, content can be null
         if let Some(tool_calls) = &m.tool_calls {
             if parts.is_empty() {
-                msg["content"] = serde_json::json!("");
+                msg["content"] = serde_json::Value::Null;
             } else {
                 msg["content"] = serde_json::json!(parts);
             }
@@ -138,15 +133,10 @@ pub async fn messages_to_openai_json_text_only(
 
         let mut msg = serde_json::json!({ "role": m.role });
 
-        // Include reasoning_content if present (DeepSeek R1/reasoner requires this in history)
-        if let Some(reasoning) = &m.reasoning_content {
-            msg["reasoning_content"] = serde_json::json!(reasoning);
-        }
-
-        // For assistant messages with tool_calls, content can be empty string
+        // For assistant messages with tool_calls, content can be null
         if let Some(tool_calls) = &m.tool_calls {
             if parts.is_empty() {
-                msg["content"] = serde_json::json!("");
+                msg["content"] = serde_json::Value::Null;
             } else {
                 msg["content"] = serde_json::json!(parts);
             }