perf(backend): 使用 gjson/sjson 优化热路径 JSON 处理

将 API 网关热路径中的 json.Unmarshal+json.Marshal 替换为 gjson 零拷贝查询和 sjson 精准写入：
- unwrapV1InternalResponse 性能提升 22x（4009ns→182ns），内存分配减少 28.5x
- unwrapGeminiResponse、extractGeminiUsage、estimateGeminiCountTokens、ParseGeminiRateLimitResetTime 改为接收 []byte 使用 gjson 提取
- ParseGatewayRequest 的 model/stream/metadata/thinking/max_tokens 改用 gjson 类型安全提取
- Handler 层（sora/openai）改用 gjson 提取字段、sjson 注入/修改字段，移除 map[string]any 中间变量
- Sora Client 响应解析改用 gjson ForEach 遍历，减少内存分配
- 新增约 100 个单元测试用例，所有改动函数覆盖率 >85%

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/antigravity_gateway_service_test.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 
@@ -889,3 +890,144 @@ func TestAntigravityClientWriter(t *testing.T) {
 		require.True(t, cw.Disconnected())
 	})
 }
+
+// TestUnwrapV1InternalResponse 测试 unwrapV1InternalResponse 的各种输入场景
+func TestUnwrapV1InternalResponse(t *testing.T) {
+	svc := &AntigravityGatewayService{}
+
+	// 构造 >50KB 的大型 JSON
+	largePadding := strings.Repeat("x", 50*1024)
+	largeInput := []byte(fmt.Sprintf(`{"response":{"id":"big","pad":"%s"}}`, largePadding))
+	largeExpected := fmt.Sprintf(`{"id":"big","pad":"%s"}`, largePadding)
+
+	tests := []struct {
+		name     string
+		input    []byte
+		expected string
+		wantErr  bool
+	}{
+		{
+			name:     "正常 response 包装",
+			input:    []byte(`{"response":{"id":"123","content":"hello"}}`),
+			expected: `{"id":"123","content":"hello"}`,
+		},
+		{
+			name:     "无 response 透传",
+			input:    []byte(`{"id":"456"}`),
+			expected: `{"id":"456"}`,
+		},
+		{
+			name:     "空 JSON",
+			input:    []byte(`{}`),
+			expected: `{}`,
+		},
+		{
+			name:     "response 为 null",
+			input:    []byte(`{"response":null}`),
+			expected: `null`,
+		},
+		{
+			name:     "response 为基础类型 string",
+			input:    []byte(`{"response":"hello"}`),
+			expected: `"hello"`,
+		},
+		{
+			name:     "非法 JSON",
+			input:    []byte(`not json`),
+			expected: `not json`,
+		},
+		{
+			name:     "嵌套 response 只解一层",
+			input:    []byte(`{"response":{"response":{"inner":true}}}`),
+			expected: `{"response":{"inner":true}}`,
+		},
+		{
+			name:     "大型 JSON >50KB",
+			input:    largeInput,
+			expected: largeExpected,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := svc.unwrapV1InternalResponse(tt.input)
+			if tt.wantErr {
+				require.Error(t, err)
+				return
+			}
+			require.NoError(t, err)
+			require.Equal(t, tt.expected, strings.TrimSpace(string(got)))
+		})
+	}
+}
+
+// --- unwrapV1InternalResponse benchmark 对照组 ---
+
+// unwrapV1InternalResponseOld 旧实现：Unmarshal+Marshal 双重开销（仅用于 benchmark 对照）
+func unwrapV1InternalResponseOld(body []byte) ([]byte, error) {
+	var outer map[string]any
+	if err := json.Unmarshal(body, &outer); err != nil {
+		return nil, err
+	}
+	if resp, ok := outer["response"]; ok {
+		return json.Marshal(resp)
+	}
+	return body, nil
+}
+
+func BenchmarkUnwrapV1Internal_Old_Small(b *testing.B) {
+	body := []byte(`{"response":{"candidates":[{"content":{"parts":[{"text":"hello world"}]}}],"usageMetadata":{"promptTokenCount":10,"candidatesTokenCount":5}}}`)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _ = unwrapV1InternalResponseOld(body)
+	}
+}
+
+func BenchmarkUnwrapV1Internal_New_Small(b *testing.B) {
+	body := []byte(`{"response":{"candidates":[{"content":{"parts":[{"text":"hello world"}]}}],"usageMetadata":{"promptTokenCount":10,"candidatesTokenCount":5}}}`)
+	svc := &AntigravityGatewayService{}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _ = svc.unwrapV1InternalResponse(body)
+	}
+}
+
+func BenchmarkUnwrapV1Internal_Old_Large(b *testing.B) {
+	body := generateLargeUnwrapJSON(10 * 1024) // ~10KB
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _ = unwrapV1InternalResponseOld(body)
+	}
+}
+
+func BenchmarkUnwrapV1Internal_New_Large(b *testing.B) {
+	body := generateLargeUnwrapJSON(10 * 1024) // ~10KB
+	svc := &AntigravityGatewayService{}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _ = svc.unwrapV1InternalResponse(body)
+	}
+}
+
+// generateLargeUnwrapJSON 生成指定最小大小的包含 response 包装的 JSON
+func generateLargeUnwrapJSON(minSize int) []byte {
+	parts := make([]map[string]string, 0)
+	current := 0
+	for current < minSize {
+		text := fmt.Sprintf("这是第 %d 段内容，用于填充 JSON 到目标大小。", len(parts)+1)
+		parts = append(parts, map[string]string{"text": text})
+		current += len(text) + 20 // 估算 JSON 编码开销
+	}
+	inner := map[string]any{
+		"candidates": []map[string]any{
+			{"content": map[string]any{"parts": parts}},
+		},
+		"usageMetadata": map[string]any{
+			"promptTokenCount":     100,
+			"candidatesTokenCount": 50,
+		},
+	}
+	outer := map[string]any{"response": inner}
+	b, _ := json.Marshal(outer)
+	return b
+}