feat(openai): 增加 gpt-5.4-mini/nano 模型支持与定价配置

- 接入 gpt-5.4-mini/nano 模型识别与规范化，补充默认模型列表
- 增加 gpt-5.4-mini/nano 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 配置
- 补充 service tier(priority/flex) 计费回归测试

backend/internal/service/openai_codex_transform_test.go

@@ -238,6 +238,10 @@ func TestNormalizeCodexModel_Gpt53(t *testing.T) {
 		"gpt-5.4-high":              "gpt-5.4",
 		"gpt-5.4-chat-latest":       "gpt-5.4",
 		"gpt 5.4":                   "gpt-5.4",
+		"gpt-5.4-mini":              "gpt-5.4-mini",
+		"gpt 5.4 mini":              "gpt-5.4-mini",
+		"gpt-5.4-nano":              "gpt-5.4-nano",
+		"gpt 5.4 nano":              "gpt-5.4-nano",
 		"gpt-5.3":                   "gpt-5.3-codex",
 		"gpt-5.3-codex":             "gpt-5.3-codex",
 		"gpt-5.3-codex-xhigh":       "gpt-5.3-codex",