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_test.go

@@ -167,6 +167,10 @@ func TestApplyCodexOAuthTransform_EmptyInput(t *testing.T) {
 
 func TestNormalizeCodexModel_Gpt53(t *testing.T) {
 	cases := map[string]string{
+		"gpt-5.4":                   "gpt-5.4",
+		"gpt-5.4-high":              "gpt-5.4",
+		"gpt-5.4-chat-latest":       "gpt-5.4",
+		"gpt 5.4":                   "gpt-5.4",
 		"gpt-5.3":                   "gpt-5.3-codex",
 		"gpt-5.3-codex":             "gpt-5.3-codex",
 		"gpt-5.3-codex-xhigh":       "gpt-5.3-codex",