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/billing_service_test.go

@@ -133,7 +133,7 @@ func TestGetModelPricing_CaseInsensitive(t *testing.T) {
 	require.Equal(t, p1.InputPricePerToken, p2.InputPricePerToken)
 }
 
-func TestGetModelPricing_UnknownModelFallsBackToSonnet(t *testing.T) {
+func TestGetModelPricing_UnknownClaudeModelFallsBackToSonnet(t *testing.T) {
 	svc := newTestBillingService()
 
 	// 不包含 opus/sonnet/haiku 关键词的 Claude 模型会走默认 Sonnet 价格
@@ -142,6 +142,71 @@ func TestGetModelPricing_UnknownModelFallsBackToSonnet(t *testing.T) {
 	require.InDelta(t, 3e-6, pricing.InputPricePerToken, 1e-12)
 }
 
+func TestGetModelPricing_UnknownOpenAIModelReturnsError(t *testing.T) {
+	svc := newTestBillingService()
+
+	pricing, err := svc.GetModelPricing("gpt-unknown-model")
+	require.Error(t, err)
+	require.Nil(t, pricing)
+	require.Contains(t, err.Error(), "pricing not found")
+}
+
+func TestGetModelPricing_OpenAIGPT51Fallback(t *testing.T) {
+	svc := newTestBillingService()
+
+	pricing, err := svc.GetModelPricing("gpt-5.1")
+	require.NoError(t, err)
+	require.NotNil(t, pricing)
+	require.InDelta(t, 1.25e-6, pricing.InputPricePerToken, 1e-12)
+}
+
+func TestGetModelPricing_OpenAIGPT54Fallback(t *testing.T) {
+	svc := newTestBillingService()
+
+	pricing, err := svc.GetModelPricing("gpt-5.4")
+	require.NoError(t, err)
+	require.NotNil(t, pricing)
+	require.InDelta(t, 2.5e-6, pricing.InputPricePerToken, 1e-12)
+	require.InDelta(t, 15e-6, pricing.OutputPricePerToken, 1e-12)
+	require.InDelta(t, 0.25e-6, pricing.CacheReadPricePerToken, 1e-12)
+}
+
+func TestGetFallbackPricing_FamilyMatching(t *testing.T) {
+	svc := newTestBillingService()
+
+	tests := []struct {
+		name             string
+		model            string
+		expectedInput    float64
+		expectNilPricing bool
+	}{
+		{name: "empty model", model: "   ", expectNilPricing: true},
+		{name: "claude opus 4.6", model: "claude-opus-4.6-20260201", expectedInput: 5e-6},
+		{name: "claude opus 4.5 alt separator", model: "claude-opus-4-5-20260101", expectedInput: 5e-6},
+		{name: "claude generic model fallback sonnet", model: "claude-foo-bar", expectedInput: 3e-6},
+		{name: "gemini explicit fallback", model: "gemini-3-1-pro", expectedInput: 2e-6},
+		{name: "gemini unknown no fallback", model: "gemini-2.0-pro", expectNilPricing: true},
+		{name: "openai gpt5.1", model: "gpt-5.1", expectedInput: 1.25e-6},
+		{name: "openai gpt5.4", model: "gpt-5.4", expectedInput: 2.5e-6},
+		{name: "openai gpt5.3 codex", model: "gpt-5.3-codex", expectedInput: 1.5e-6},
+		{name: "openai gpt5.1 codex max alias", model: "gpt-5.1-codex-max", expectedInput: 1.5e-6},
+		{name: "openai codex mini latest alias", model: "codex-mini-latest", expectedInput: 1.5e-6},
+		{name: "openai unknown no fallback", model: "gpt-unknown-model", expectNilPricing: true},
+		{name: "non supported family", model: "qwen-max", expectNilPricing: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			pricing := svc.getFallbackPricing(tt.model)
+			if tt.expectNilPricing {
+				require.Nil(t, pricing)
+				return
+			}
+			require.NotNil(t, pricing)
+			require.InDelta(t, tt.expectedInput, pricing.InputPricePerToken, 1e-12)
+		})
+	}
+}
 func TestCalculateCostWithLongContext_BelowThreshold(t *testing.T) {
 	svc := newTestBillingService()