fix: extract and log Claude output_config.effort in usage records

Claude's output_config.effort parameter (low/medium/high/max) was not
being extracted from requests or logged in the reasoning_effort column
of usage logs. Only the OpenAI path populated this field.

Changes:
- Extract output_config.effort in ParseGatewayRequest
- Add ReasoningEffort field to ForwardResult
- Populate reasoning_effort in both RecordUsage and RecordUsageWithLongContext
- Guard against overwriting service-set effort values in handler
- Update stale comments that described reasoning_effort as OpenAI-only
- Add unit tests for extraction, normalization, and persistence

backend/internal/service/gateway_request.go

@@ -60,6 +60,7 @@ type ParsedRequest struct {
 	Messages        []any           // messages 数组
 	HasSystem       bool            // 是否包含 system 字段（包含 null 也视为显式传入）
 	ThinkingEnabled bool            // 是否开启 thinking（部分平台会影响最终模型名）
+	OutputEffort    string          // output_config.effort（Claude API 的推理强度控制）
 	MaxTokens       int             // max_tokens 值（用于探测请求拦截）
 	SessionContext  *SessionContext // 可选：请求上下文区分因子（nil 时行为不变）
 
@@ -116,6 +117,9 @@ func ParseGatewayRequest(body []byte, protocol string) (*ParsedRequest, error) {
 		parsed.ThinkingEnabled = true
 	}
 
+	// output_config.effort: Claude API 的推理强度控制参数
+	parsed.OutputEffort = strings.TrimSpace(gjson.Get(jsonStr, "output_config.effort").String())
+
 	// max_tokens: 仅接受整数值
 	maxTokensResult := gjson.Get(jsonStr, "max_tokens")
 	if maxTokensResult.Exists() && maxTokensResult.Type == gjson.Number {
@@ -747,6 +751,21 @@ func filterThinkingBlocksInternal(body []byte, _ bool) []byte {
 	return newBody
 }
 
+// NormalizeClaudeOutputEffort normalizes Claude's output_config.effort value.
+// Returns nil for empty or unrecognized values.
+func NormalizeClaudeOutputEffort(raw string) *string {
+	value := strings.ToLower(strings.TrimSpace(raw))
+	if value == "" {
+		return nil
+	}
+	switch value {
+	case "low", "medium", "high", "max":
+		return &value
+	default:
+		return nil
+	}
+}
+
 // =========================
 // Thinking Budget Rectifier
 // =========================