fix: openai默认模型新增gpt5.5

Author: Wei-Shaw

backend/internal/pkg/openai/constants.go

@@ -15,6 +15,7 @@ type Model struct {
 
 // DefaultModels OpenAI models list
 var DefaultModels = []Model{
+	{ID: "gpt-5.5", Object: "model", Created: 1776873600, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.5"},
 	{ID: "gpt-5.4", Object: "model", Created: 1738368000, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.4"},
 	{ID: "gpt-5.4-mini", Object: "model", Created: 1738368000, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.4 Mini"},
 	{ID: "gpt-5.3-codex", Object: "model", Created: 1735689600, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.3 Codex"},