fix: Correct JSON field name for SystemInstructions in GeminiChatRequest

- Updated the JSON field name from "system_instructions" to "system_instruction" to ensure consistency and accuracy in the data structure.

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_instructions,omitempty"`
+	SystemInstructions *GeminiPart                `json:"system_instruction,omitempty"`
 }
 
 type GeminiInlineData struct {