feat(openai): 增加 gpt-5.4 模型支持与定价配置

- 接入 gpt-5.4 模型识别与规范化，补充默认模型列表
- 增加 gpt-5.4 输入/缓存命中/输出价格与计费兜底逻辑
- 同步前端模型白名单与 OpenCode 上下文窗口（1050000/128000）

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
(cherry picked from commit 924476dcac6181cd0f3ee731ec7b73672ff03793)

backend/internal/service/openai_codex_transform.go

@@ -9,6 +9,13 @@ import (
 var codexCLIInstructions string
 
 var codexModelMap = map[string]string{
+	"gpt-5.4":                    "gpt-5.4",
+	"gpt-5.4-none":               "gpt-5.4",
+	"gpt-5.4-low":                "gpt-5.4",
+	"gpt-5.4-medium":             "gpt-5.4",
+	"gpt-5.4-high":               "gpt-5.4",
+	"gpt-5.4-xhigh":              "gpt-5.4",
+	"gpt-5.4-chat-latest":        "gpt-5.4",
 	"gpt-5.3":                    "gpt-5.3-codex",
 	"gpt-5.3-none":               "gpt-5.3-codex",
 	"gpt-5.3-low":                "gpt-5.3-codex",
@@ -154,6 +161,9 @@ func normalizeCodexModel(model string) string {
 
 	normalized := strings.ToLower(modelID)
 
+	if strings.Contains(normalized, "gpt-5.4") || strings.Contains(normalized, "gpt 5.4") {
+		return "gpt-5.4"
+	}
 	if strings.Contains(normalized, "gpt-5.2-codex") || strings.Contains(normalized, "gpt 5.2 codex") {
 		return "gpt-5.2-codex"
 	}