feat(api): add tuning params (temperature, top_p, penalties, stop) to MessageRequest

MessageRequest was missing standard OpenAI-compatible generation tuning
parameters. Callers had no way to control temperature, top_p,
frequency_penalty, presence_penalty, or stop sequences.

Changes:
- Added 5 optional fields to MessageRequest (all Option, None by default)
- Wired into build_chat_completion_request: only included in payload when set
- All existing construction sites updated with ..Default::default()
- MessageRequest now derives Default for ergonomic partial construction

Tests added:
- tuning_params_included_in_payload_when_set: all 5 params flow into JSON
- tuning_params_omitted_from_payload_when_none: absent params stay absent

83 api lib tests passing, 0 failing.
cargo check --workspace: 0 warnings.

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

@@ -721,6 +721,25 @@ fn build_chat_completion_request(request: &MessageRequest, config: OpenAiCompatC
         payload["tool_choice"] = openai_tool_choice(tool_choice);
     }
 
+    // OpenAI-compatible tuning parameters — only included when explicitly set.
+    if let Some(temperature) = request.temperature {
+        payload["temperature"] = json!(temperature);
+    }
+    if let Some(top_p) = request.top_p {
+        payload["top_p"] = json!(top_p);
+    }
+    if let Some(frequency_penalty) = request.frequency_penalty {
+        payload["frequency_penalty"] = json!(frequency_penalty);
+    }
+    if let Some(presence_penalty) = request.presence_penalty {
+        payload["presence_penalty"] = json!(presence_penalty);
+    }
+    if let Some(stop) = &request.stop {
+        if !stop.is_empty() {
+            payload["stop"] = json!(stop);
+        }
+    }
+
     payload
 }
 
@@ -1049,6 +1068,7 @@ mod tests {
                 }]),
                 tool_choice: Some(ToolChoice::Auto),
                 stream: false,
+                ..Default::default()
             },
             OpenAiCompatConfig::xai(),
         );
@@ -1071,6 +1091,7 @@ mod tests {
                 tools: None,
                 tool_choice: None,
                 stream: true,
+            ..Default::default()
             },
             OpenAiCompatConfig::openai(),
         );
@@ -1089,6 +1110,7 @@ mod tests {
                 tools: None,
                 tool_choice: None,
                 stream: true,
+            ..Default::default()
             },
             OpenAiCompatConfig::xai(),
         );
@@ -1159,4 +1181,45 @@ mod tests {
         assert_eq!(normalize_finish_reason("stop"), "end_turn");
         assert_eq!(normalize_finish_reason("tool_calls"), "tool_use");
     }
+
+    #[test]
+    fn tuning_params_included_in_payload_when_set() {
+        let request = MessageRequest {
+            model: "gpt-4o".to_string(),
+            max_tokens: 1024,
+            messages: vec![],
+            system: None,
+            tools: None,
+            tool_choice: None,
+            stream: false,
+            temperature: Some(0.7),
+            top_p: Some(0.9),
+            frequency_penalty: Some(0.5),
+            presence_penalty: Some(0.3),
+            stop: Some(vec!["\n".to_string()]),
+        };
+        let payload = build_chat_completion_request(&request, OpenAiCompatConfig::openai());
+        assert_eq!(payload["temperature"], 0.7);
+        assert_eq!(payload["top_p"], 0.9);
+        assert_eq!(payload["frequency_penalty"], 0.5);
+        assert_eq!(payload["presence_penalty"], 0.3);
+        assert_eq!(payload["stop"], json!(["\n"]));
+    }
+
+    #[test]
+    fn tuning_params_omitted_from_payload_when_none() {
+        let request = MessageRequest {
+            model: "gpt-4o".to_string(),
+            max_tokens: 1024,
+            messages: vec![],
+            stream: false,
+            ..Default::default()
+        };
+        let payload = build_chat_completion_request(&request, OpenAiCompatConfig::openai());
+        assert!(payload.get("temperature").is_none(), "temperature should be absent");
+        assert!(payload.get("top_p").is_none(), "top_p should be absent");
+        assert!(payload.get("frequency_penalty").is_none());
+        assert!(payload.get("presence_penalty").is_none());
+        assert!(payload.get("stop").is_none());
+    }
 }