test(billing): tighten DeepSeek V4 fallback assertions; clarify branch comments

Address copilot-pull-request-reviewer feedback on #2157:

- billing_service_test.go: extend TestGetFallbackPricing_FamilyMatching
  with optional expectedOutput / expectedCacheRead fields and assert
  full Input/Output/CacheRead pricing for all 4 DeepSeek cases
  (v4-pro, v4-flash, deepseek-chat→flash, deepseek-reasoner→flash),
  preventing silent regression to 0 output/cache cost.
- billing_service.go: rewrite the DeepSeek block comment to explicitly
  describe its scope (V4 Pro/Flash + chat/reasoner aliases, no
  unknown-deepseek fallback) and tighten the OpenAI comment to make
  it unambiguous that it only describes the OpenAI/Codex branch
  immediately below it.

backend/internal/service/billing_service.go

@@ -362,19 +362,19 @@ func (s *BillingService) getFallbackPricing(model string) *ModelPricing {
 		return s.fallbackPrices["gemini-3.1-pro"]
 	}
 
-	// DeepSeek V4 系列
+	// DeepSeek V4 系列：仅匹配已知 V4 Pro/Flash 与官方兼容别名
+	// （deepseek-chat / deepseek-reasoner → V4 Flash），未知 deepseek-* 型号不回退，避免误计价。
 	if strings.Contains(modelLower, "deepseek-v4-flash") {
 		return s.fallbackPrices["deepseek-v4-flash"]
 	}
 	if strings.Contains(modelLower, "deepseek-v4-pro") {
 		return s.fallbackPrices["deepseek-v4-pro"]
 	}
-	// deepseek-chat / deepseek-reasoner → V4 Flash（官方兼容别名）
 	if strings.Contains(modelLower, "deepseek-chat") || strings.Contains(modelLower, "deepseek-reasoner") {
 		return s.fallbackPrices["deepseek-v4-flash"]
 	}
 
-	// OpenAI 仅匹配已知 GPT-5/Codex 族，避免未知 OpenAI 型号误计价。
+	// OpenAI（GPT-5 / Codex 族）：仅匹配已知型号，避免未知 OpenAI 型号误计价。
 	if normalized := normalizeKnownOpenAICodexModel(modelLower); normalized != "" {
 		switch normalized {
 		case "gpt-5.5":

backend/internal/service/billing_service_test.go

@@ -332,11 +332,14 @@ func TestCalculateCost_LongContextAppliesMultiplierToCacheCreation5mAnd1h(t *tes
 func TestGetFallbackPricing_FamilyMatching(t *testing.T) {
 	svc := newTestBillingService()
 
+	// expectedOutput / expectedCacheRead 为 0 时跳过该字段断言（保持与原有用例兼容）。
 	tests := []struct {
-		name             string
-		model            string
-		expectedInput    float64
-		expectNilPricing bool
+		name              string
+		model             string
+		expectedInput     float64
+		expectedOutput    float64
+		expectedCacheRead float64
+		expectNilPricing  bool
 	}{
 		{name: "empty model", model: "   ", expectNilPricing: true},
 		{name: "claude opus 4.6", model: "claude-opus-4.6-20260201", expectedInput: 5e-6},
@@ -352,10 +355,34 @@ func TestGetFallbackPricing_FamilyMatching(t *testing.T) {
 		{name: "openai legacy gpt5.1 codex falls back to gpt5.3 codex", model: "gpt-5.1-codex", expectedInput: 1.5e-6},
 		{name: "openai legacy codex mini latest falls back to gpt5.3 codex", model: "codex-mini-latest", expectedInput: 1.5e-6},
 		{name: "openai unknown no fallback", model: "gpt-unknown-model", expectNilPricing: true},
-		{name: "deepseek v4 pro", model: "deepseek-v4-pro", expectedInput: 4.35e-7},
-		{name: "deepseek v4 flash", model: "deepseek-v4-flash", expectedInput: 1.4e-7},
-		{name: "deepseek chat alias → flash", model: "deepseek-chat", expectedInput: 1.4e-7},
-		{name: "deepseek reasoner alias → flash", model: "deepseek-reasoner", expectedInput: 1.4e-7},
+		{
+			name:              "deepseek v4 pro",
+			model:             "deepseek-v4-pro",
+			expectedInput:     4.35e-7,
+			expectedOutput:    8.7e-7,
+			expectedCacheRead: 3.625e-9,
+		},
+		{
+			name:              "deepseek v4 flash",
+			model:             "deepseek-v4-flash",
+			expectedInput:     1.4e-7,
+			expectedOutput:    2.8e-7,
+			expectedCacheRead: 2.8e-9,
+		},
+		{
+			name:              "deepseek chat alias → flash",
+			model:             "deepseek-chat",
+			expectedInput:     1.4e-7,
+			expectedOutput:    2.8e-7,
+			expectedCacheRead: 2.8e-9,
+		},
+		{
+			name:              "deepseek reasoner alias → flash",
+			model:             "deepseek-reasoner",
+			expectedInput:     1.4e-7,
+			expectedOutput:    2.8e-7,
+			expectedCacheRead: 2.8e-9,
+		},
 		{name: "non supported family", model: "qwen-max", expectNilPricing: true},
 	}
 
@@ -368,6 +395,14 @@ func TestGetFallbackPricing_FamilyMatching(t *testing.T) {
 			}
 			require.NotNil(t, pricing)
 			require.InDelta(t, tt.expectedInput, pricing.InputPricePerToken, 1e-12)
+			if tt.expectedOutput != 0 {
+				require.InDelta(t, tt.expectedOutput, pricing.OutputPricePerToken, 1e-12,
+					"OutputPricePerToken mismatch for %s", tt.model)
+			}
+			if tt.expectedCacheRead != 0 {
+				require.InDelta(t, tt.expectedCacheRead, pricing.CacheReadPricePerToken, 1e-14,
+					"CacheReadPricePerToken mismatch for %s", tt.model)
+			}
 		})
 	}
 }