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/pricing_service.go

@@ -34,6 +34,22 @@ var (
 		Mode:                            "chat",
 		SupportsPromptCaching:           true,
 	}
+	openAIGPT54MiniFallbackPricing = &LiteLLMModelPricing{
+		InputCostPerToken:       7.5e-07,
+		OutputCostPerToken:      4.5e-06,
+		CacheReadInputTokenCost: 7.5e-08,
+		LiteLLMProvider:         "openai",
+		Mode:                    "chat",
+		SupportsPromptCaching:   true,
+	}
+	openAIGPT54NanoFallbackPricing = &LiteLLMModelPricing{
+		InputCostPerToken:       2e-07,
+		OutputCostPerToken:      1.25e-06,
+		CacheReadInputTokenCost: 2e-08,
+		LiteLLMProvider:         "openai",
+		Mode:                    "chat",
+		SupportsPromptCaching:   true,
+	}
 )
 
 // LiteLLMModelPricing LiteLLM价格数据结构
@@ -723,6 +739,18 @@ func (s *PricingService) matchOpenAIModel(model string) *LiteLLMModelPricing {
 		}
 	}
 
+	if strings.HasPrefix(model, "gpt-5.4-mini") {
+		logger.With(zap.String("component", "service.pricing")).
+			Info(fmt.Sprintf("[Pricing] OpenAI fallback matched %s -> %s", model, "gpt-5.4-mini(static)"))
+		return openAIGPT54MiniFallbackPricing
+	}
+
+	if strings.HasPrefix(model, "gpt-5.4-nano") {
+		logger.With(zap.String("component", "service.pricing")).
+			Info(fmt.Sprintf("[Pricing] OpenAI fallback matched %s -> %s", model, "gpt-5.4-nano(static)"))
+		return openAIGPT54NanoFallbackPricing
+	}
+
 	if strings.HasPrefix(model, "gpt-5.4") {
 		logger.With(zap.String("component", "service.pricing")).
 			Info(fmt.Sprintf("[Pricing] OpenAI fallback matched %s -> %s", model, "gpt-5.4(static)"))