fix(translator/gemini): support developer role in OpenAI Responses requests

internal/translator/gemini/openai/responses/gemini_openai-responses_request.go

@@ -118,7 +118,7 @@ func ConvertOpenAIResponsesRequestToGemini(modelName string, inputRawJSON []byte
 
 			switch itemType {
 			case "message":
-				if strings.EqualFold(itemRole, "system") {
+				if strings.EqualFold(itemRole, "system") || strings.EqualFold(itemRole, "developer") {
 					if contentArray := item.Get("content"); contentArray.Exists() {
 						systemInstr := []byte(`{"parts":[]}`)
 						if systemInstructionResult := gjson.GetBytes(out, "systemInstruction"); systemInstructionResult.Exists() {

internal/translator/gemini/openai/responses/gemini_openai-responses_request_test.go

@@ -0,0 +1,109 @@
+package responses
+
+import (
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestConvertOpenAIResponsesRequestToGemini_SystemAndDeveloperRoles(t *testing.T) {
+	// Test system role conversion
+	systemInput := []byte(`{
+		"instructions": "Be a helpful assistant",
+		"input": [
+			{
+				"type": "message",
+				"role": "system",
+				"content": [
+					{
+						"type": "input_text",
+						"text": "System message text"
+					}
+				]
+			},
+			{
+				"type": "message",
+				"role": "user",
+				"content": [
+					{
+						"type": "input_text",
+						"text": "Hello"
+					}
+				]
+			}
+		]
+	}`)
+
+	outSystem := ConvertOpenAIResponsesRequestToGemini("gemini-3.5-flash", systemInput, false)
+	resSystem := gjson.ParseBytes(outSystem)
+
+	systemInstruction := resSystem.Get("systemInstruction")
+	if !systemInstruction.Exists() {
+		t.Errorf("Expected systemInstruction field to exist")
+	}
+	parts := systemInstruction.Get("parts")
+	if parts.Get("#").Int() != 2 {
+		t.Errorf("Expected 2 parts in systemInstruction, got %d", parts.Get("#").Int())
+	}
+	if parts.Get("0.text").String() != "Be a helpful assistant" {
+		t.Errorf("Expected first part to be 'Be a helpful assistant', got '%s'", parts.Get("0.text").String())
+	}
+	if parts.Get("1.text").String() != "System message text" {
+		t.Errorf("Expected second part to be 'System message text', got '%s'", parts.Get("1.text").String())
+	}
+
+	// Test developer role conversion (which is the main bug we're addressing)
+	developerInput := []byte(`{
+		"instructions": "Be a helpful assistant",
+		"input": [
+			{
+				"type": "message",
+				"role": "developer",
+				"content": [
+					{
+						"type": "input_text",
+						"text": "Developer message text"
+					}
+				]
+			},
+			{
+				"type": "message",
+				"role": "user",
+				"content": [
+					{
+						"type": "input_text",
+						"text": "Hello"
+					}
+				]
+			}
+		]
+	}`)
+
+	outDev := ConvertOpenAIResponsesRequestToGemini("gemini-3.5-flash", developerInput, false)
+	resDev := gjson.ParseBytes(outDev)
+
+	systemInstructionDev := resDev.Get("systemInstruction")
+	if !systemInstructionDev.Exists() {
+		t.Errorf("Expected systemInstruction field to exist for developer role")
+	}
+	partsDev := systemInstructionDev.Get("parts")
+	if partsDev.Get("#").Int() != 2 {
+		t.Errorf("Expected 2 parts in systemInstruction for developer role, got %d", partsDev.Get("#").Int())
+	}
+	if partsDev.Get("0.text").String() != "Be a helpful assistant" {
+		t.Errorf("Expected first part to be 'Be a helpful assistant', got '%s'", partsDev.Get("0.text").String())
+	}
+	if partsDev.Get("1.text").String() != "Developer message text" {
+		t.Errorf("Expected second part to be 'Developer message text', got '%s'", partsDev.Get("1.text").String())
+	}
+
+	// Ensure role 'developer' is not sent inside contents array as a regular message
+	contents := resDev.Get("contents")
+	contents.ForEach(func(_, value gjson.Result) bool {
+		role := value.Get("role").String()
+		if role == "developer" {
+			t.Errorf("Role 'developer' leaked into contents array")
+		}
+		return true
+	})
+}