refactor: Update SystemInstructions type in GeminiChatRequest and adjust handling in CovertGemini2OpenAI

- Changed SystemInstructions from *GeminiPart to *GeminiChatContent in GeminiChatRequest for improved structure.
- Updated CovertGemini2OpenAI function to accommodate the new SystemInstructions type, ensuring proper handling of message content.

relay/channel/gemini/dto.go

@@ -5,7 +5,7 @@ type GeminiChatRequest struct {
 	SafetySettings     []GeminiChatSafetySettings `json:"safety_settings,omitempty"`
 	GenerationConfig   GeminiChatGenerationConfig `json:"generation_config,omitempty"`
 	Tools              []GeminiChatTools          `json:"tools,omitempty"`
-	SystemInstructions *GeminiPart                `json:"system_instruction,omitempty"`
+	SystemInstructions *GeminiChatContent         `json:"system_instruction,omitempty"`
 }
 
 type GeminiInlineData struct {

relay/channel/gemini/relay-gemini.go

@@ -76,8 +76,12 @@ func CovertGemini2OpenAI(textRequest dto.GeneralOpenAIRequest) *GeminiChatReques
 	for _, message := range textRequest.Messages {
 
 		if message.Role == "system" {
-			geminiRequest.SystemInstructions = &GeminiPart{
-				Text: message.StringContent(),
+			geminiRequest.SystemInstructions = &GeminiChatContent{
+				Parts: []GeminiPart{
+					{
+						Text: message.StringContent(),
+					},
+				},
 			}
 			continue
 		}