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/pricing_service_test.go

@@ -51,3 +51,17 @@ func TestGetModelPricing_OpenAIFallbackMatchedLoggedAsInfo(t *testing.T) {
 	require.True(t, logSink.ContainsMessageAtLevel("[Pricing] OpenAI fallback matched gpt-5.3-codex -> gpt-5.2-codex", "info"))
 	require.False(t, logSink.ContainsMessageAtLevel("[Pricing] OpenAI fallback matched gpt-5.3-codex -> gpt-5.2-codex", "warn"))
 }
+
+func TestGetModelPricing_Gpt54UsesStaticFallbackWhenRemoteMissing(t *testing.T) {
+	svc := &PricingService{
+		pricingData: map[string]*LiteLLMModelPricing{
+			"gpt-5.1-codex": &LiteLLMModelPricing{InputCostPerToken: 1.25e-6},
+		},
+	}
+
+	got := svc.GetModelPricing("gpt-5.4")
+	require.NotNil(t, got)
+	require.InDelta(t, 2.5e-6, got.InputCostPerToken, 1e-12)
+	require.InDelta(t, 1.5e-5, got.OutputCostPerToken, 1e-12)
+	require.InDelta(t, 2.5e-7, got.CacheReadInputTokenCost, 1e-12)
+}