fix(openai): 修复 gpt-5.4 长上下文计费与快照白名单

补齐 gpt-5.4 fallback 的长上下文计费元信息，\n确保超过 272000 输入 token 时对整次会话应用\n2x 输入与 1.5x 输出计费规则。\n\n同时将官方快照 gpt-5.4-2026-03-05 加入前端\n白名单候选与回归测试，避免 whitelist 模式误拦截。\n\nCo-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

(cherry picked from commit d95497af87f608c6dadcbe7d6e851de9413ae147)

backend/internal/service/billing_service_test.go

@@ -169,6 +169,28 @@ func TestGetModelPricing_OpenAIGPT54Fallback(t *testing.T) {
 	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)
+	require.Equal(t, 272000, pricing.LongContextInputThreshold)
+	require.InDelta(t, 2.0, pricing.LongContextInputMultiplier, 1e-12)
+	require.InDelta(t, 1.5, pricing.LongContextOutputMultiplier, 1e-12)
+}
+
+func TestCalculateCost_OpenAIGPT54LongContextAppliesWholeSessionMultipliers(t *testing.T) {
+	svc := newTestBillingService()
+
+	tokens := UsageTokens{
+		InputTokens:  300000,
+		OutputTokens: 4000,
+	}
+
+	cost, err := svc.CalculateCost("gpt-5.4-2026-03-05", tokens, 1.0)
+	require.NoError(t, err)
+
+	expectedInput := float64(tokens.InputTokens) * 2.5e-6 * 2.0
+	expectedOutput := float64(tokens.OutputTokens) * 15e-6 * 1.5
+	require.InDelta(t, expectedInput, cost.InputCost, 1e-10)
+	require.InDelta(t, expectedOutput, cost.OutputCost, 1e-10)
+	require.InDelta(t, expectedInput+expectedOutput, cost.TotalCost, 1e-10)
+	require.InDelta(t, expectedInput+expectedOutput, cost.ActualCost, 1e-10)
 }
 
 func TestGetFallbackPricing_FamilyMatching(t *testing.T) {