fix(providers): handle tool messages in text_only message converter

messages_to_openai_json_text_only() was missing tool-calling support,
causing DeepSeek 400 errors when conversations included tool turns.
Now mirrors messages_to_openai_json() logic for tool-role messages
(tool_call_id, name) and assistant tool_calls, with images replaced
by "[Image]" text.

src/providers/helpers.rs

@@ -84,11 +84,41 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
 /// Convert messages to OpenAI-compatible JSON, but replace images with a
 /// text placeholder "[Image]". Useful for providers that don't support
 /// multimodal in streaming mode or at all.
+///
+/// Handles tool-calling messages identically to `messages_to_openai_json`:
+/// assistant messages with `tool_calls`, and tool-role messages with
+/// `tool_call_id`/`name`.
 pub async fn messages_to_openai_json_text_only(
     messages: &[UnifiedMessage],
 ) -> Result<Vec<serde_json::Value>, AppError> {
     let mut result = Vec::new();
     for m in messages {
+        // Tool-role messages: { role: "tool", content: "...", tool_call_id: "...", name: "..." }
+        if m.role == "tool" {
+            let text_content = m
+                .content
+                .first()
+                .map(|p| match p {
+                    ContentPart::Text { text } => text.clone(),
+                    ContentPart::Image(_) => "[Image]".to_string(),
+                })
+                .unwrap_or_default();
+
+            let mut msg = serde_json::json!({
+                "role": "tool",
+                "content": text_content
+            });
+            if let Some(tool_call_id) = &m.tool_call_id {
+                msg["tool_call_id"] = serde_json::json!(tool_call_id);
+            }
+            if let Some(name) = &m.name {
+                msg["name"] = serde_json::json!(name);
+            }
+            result.push(msg);
+            continue;
+        }
+
+        // Build content parts for non-tool messages (images become "[Image]" text)
         let mut parts = Vec::new();
         for p in &m.content {
             match p {
@@ -100,10 +130,26 @@ pub async fn messages_to_openai_json_text_only(
                 }
             }
         }
-        result.push(serde_json::json!({
-            "role": m.role,
-            "content": parts
-        }));
+
+        let mut msg = serde_json::json!({ "role": m.role });
+
+        // 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::Value::Null;
+            } else {
+                msg["content"] = serde_json::json!(parts);
+            }
+            msg["tool_calls"] = serde_json::json!(tool_calls);
+        } else {
+            msg["content"] = serde_json::json!(parts);
+        }
+
+        if let Some(name) = &m.name {
+            msg["name"] = serde_json::json!(name);
+        }
+
+        result.push(msg);
     }
     Ok(result)
 }