fix(openai): flatten tools and tool_choice for Responses API

- Map nested 'function' object to top-level fields
- Support string and object-based 'tool_choice' formats
- Fix 400 Bad Request 'Missing required parameter: tools[0].name'

src/providers/openai.rs

@@ -226,10 +226,33 @@ impl super::Provider for OpenAIProvider {
         }
 
         if let Some(tools) = &request.tools {
-            body["tools"] = serde_json::json!(tools);
+            let flattened: Vec<serde_json::Value> = tools.iter().map(|t| {
+                let mut obj = serde_json::json!({
+                    "type": t.tool_type,
+                    "name": t.function.name,
+                });
+                if let Some(desc) = &t.function.description {
+                    obj["description"] = serde_json::json!(desc);
+                }
+                if let Some(params) = &t.function.parameters {
+                    obj["parameters"] = params.clone();
+                }
+                obj
+            }).collect();
+            body["tools"] = serde_json::json!(flattened);
         }
         if let Some(tool_choice) = &request.tool_choice {
-            body["tool_choice"] = serde_json::json!(tool_choice);
+            match tool_choice {
+                crate::models::ToolChoice::Mode(mode) => {
+                    body["tool_choice"] = serde_json::json!(mode);
+                }
+                crate::models::ToolChoice::Specific(specific) => {
+                    body["tool_choice"] = serde_json::json!({
+                        "type": specific.choice_type,
+                        "name": specific.function.name,
+                    });
+                }
+            }
         }
 
         let resp = self
@@ -574,10 +597,33 @@ impl super::Provider for OpenAIProvider {
         }
 
         if let Some(tools) = &request.tools {
-            body["tools"] = serde_json::json!(tools);
+            let flattened: Vec<serde_json::Value> = tools.iter().map(|t| {
+                let mut obj = serde_json::json!({
+                    "type": t.tool_type,
+                    "name": t.function.name,
+                });
+                if let Some(desc) = &t.function.description {
+                    obj["description"] = serde_json::json!(desc);
+                }
+                if let Some(params) = &t.function.parameters {
+                    obj["parameters"] = params.clone();
+                }
+                obj
+            }).collect();
+            body["tools"] = serde_json::json!(flattened);
         }
         if let Some(tool_choice) = &request.tool_choice {
-            body["tool_choice"] = serde_json::json!(tool_choice);
+            match tool_choice {
+                crate::models::ToolChoice::Mode(mode) => {
+                    body["tool_choice"] = serde_json::json!(mode);
+                }
+                crate::models::ToolChoice::Specific(specific) => {
+                    body["tool_choice"] = serde_json::json!({
+                        "type": specific.choice_type,
+                        "name": specific.function.name,
+                    });
+                }
+            }
         }
 
         let url = format!("{}/responses", self.config.base_url);