fix(api): omit tool_calls field from assistant messages when empty

When serializing a multi-turn conversation for the OpenAI-compatible path,
assistant messages with no tool calls were always emitting 'tool_calls: []'.
Some providers reject requests where a prior assistant turn carries an
explicit empty tool_calls array (400 on subsequent turns after a plain
text assistant response).

Fix: only include 'tool_calls' in the serialized assistant message when
the vec is non-empty. Empty case omits the field entirely.

This is a companion fix to fd7aade (null tool_calls in stream delta).
The two bugs are symmetric: fd7aade handled inbound null -> empty vec;
this handles outbound empty vec -> field omitted.

Two regression tests added:
- assistant_message_without_tool_calls_omits_tool_calls_field
- assistant_message_with_tool_calls_includes_tool_calls_field

115 api tests pass. Fmt clean.

Source: gaebal-gajae repro 2026-04-09 (400 on multi-turn, companion to
null tool_calls stream-delta fix).

rust/crates/api/src/providers/openai_compat.rs

@@ -853,11 +853,16 @@ fn translate_message(message: &InputMessage) -> Vec<Value> {
             if text.is_empty() && tool_calls.is_empty() {
                 Vec::new()
             } else {
-                vec![json!({
+                let mut msg = serde_json::json!({
                     "role": "assistant",
                     "content": (!text.is_empty()).then_some(text),
-                    "tool_calls": tool_calls,
-                })]
+                });
+                // Only include tool_calls when non-empty: some providers reject
+                // assistant messages with an explicit empty tool_calls array.
+                if !tool_calls.is_empty() {
+                    msg["tool_calls"] = json!(tool_calls);
+                }
+                vec![msg]
             }
         }
         _ => message
@@ -1526,6 +1531,73 @@ mod tests {
         );
     }
 
+    #[test]
+    /// Regression: when building a multi-turn request where a prior assistant
+    /// turn has no tool calls, the serialized assistant message must NOT include
+    /// `tool_calls: []`. Some providers reject requests that carry an empty
+    /// tool_calls array on assistant turns (gaebal-gajae repro 2026-04-09).
+    #[test]
+    fn assistant_message_without_tool_calls_omits_tool_calls_field() {
+        use crate::types::{InputContentBlock, InputMessage};
+
+        let request = MessageRequest {
+            model: "gpt-4o".to_string(),
+            max_tokens: 100,
+            messages: vec![InputMessage {
+                role: "assistant".to_string(),
+                content: vec![InputContentBlock::Text {
+                    text: "Hello".to_string(),
+                }],
+            }],
+            stream: false,
+            ..Default::default()
+        };
+        let payload = build_chat_completion_request(&request, OpenAiCompatConfig::openai());
+        let messages = payload["messages"].as_array().unwrap();
+        let assistant_msg = messages
+            .iter()
+            .find(|m| m["role"] == "assistant")
+            .expect("assistant message must be present");
+        assert!(
+            assistant_msg.get("tool_calls").is_none(),
+            "assistant message without tool calls must omit tool_calls field: {:?}",
+            assistant_msg
+        );
+    }
+
+    /// Regression: assistant messages WITH tool calls must still include
+    /// the tool_calls array (normal multi-turn tool-use flow).
+    #[test]
+    fn assistant_message_with_tool_calls_includes_tool_calls_field() {
+        use crate::types::{InputContentBlock, InputMessage};
+
+        let request = MessageRequest {
+            model: "gpt-4o".to_string(),
+            max_tokens: 100,
+            messages: vec![InputMessage {
+                role: "assistant".to_string(),
+                content: vec![InputContentBlock::ToolUse {
+                    id: "call_1".to_string(),
+                    name: "read_file".to_string(),
+                    input: serde_json::json!({"path": "/tmp/test"}),
+                }],
+            }],
+            stream: false,
+            ..Default::default()
+        };
+        let payload = build_chat_completion_request(&request, OpenAiCompatConfig::openai());
+        let messages = payload["messages"].as_array().unwrap();
+        let assistant_msg = messages
+            .iter()
+            .find(|m| m["role"] == "assistant")
+            .expect("assistant message must be present");
+        let tool_calls = assistant_msg
+            .get("tool_calls")
+            .expect("assistant message with tool calls must include tool_calls field");
+        assert!(tool_calls.is_array());
+        assert_eq!(tool_calls.as_array().unwrap().len(), 1);
+    }
+
     #[test]
     fn non_gpt5_uses_max_tokens() {
         // Older OpenAI models expect `max_tokens`; verify gpt-4o is unaffected.