feat: enhance ConvertGeminiRequest to set default role and handle YouTube video MIME type

relay/channel/vertex/adaptor.go

@@ -45,7 +45,8 @@ type Adaptor struct {
 }
 
 func (a *Adaptor) ConvertGeminiRequest(c *gin.Context, info *relaycommon.RelayInfo, request *dto.GeminiChatRequest) (any, error) {
-	return request, nil
+	geminiAdaptor := gemini.Adaptor{}
+	return geminiAdaptor.ConvertGeminiRequest(c, info, request)
 }
 
 func (a *Adaptor) ConvertClaudeRequest(c *gin.Context, info *relaycommon.RelayInfo, request *dto.ClaudeRequest) (any, error) {