fix: correct log levels for thinking block signature retry flow

LegacyPrintf uses inferStdLogLevel() to infer log level from message
text. Any message containing the word "error" is classified as ERROR
level, causing the entire signature-retry recovery flow (which succeeds)
to produce spurious ERROR log entries.

Changes:
- Remove noisy [SignatureCheck] debug logs inside isThinkingBlockSignatureError
  that were logging every detected signature check as ERROR
- Change retry-start log to WARN level via [warn] prefix
- Change retry-success log to INFO level by removing "error" from message

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

backend/internal/service/gateway_service.go

@@ -4197,7 +4197,7 @@ func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *A
 						resp.Body = io.NopCloser(bytes.NewReader(respBody))
 						break
 					}
-					logger.LegacyPrintf("service.gateway", "Account %d: detected thinking block signature error, retrying with filtered thinking blocks", account.ID)
+					logger.LegacyPrintf("service.gateway", "[warn] Account %d: thinking blocks have invalid signature, retrying with filtered blocks", account.ID)
 
 					// Conservative two-stage fallback:
 					// 1) Disable thinking + thinking->text (preserve content)
@@ -4212,7 +4212,7 @@ func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *A
 						retryResp, retryErr := s.httpUpstream.DoWithTLS(retryReq, proxyURL, account.ID, account.Concurrency, account.IsTLSFingerprintEnabled())
 						if retryErr == nil {
 							if retryResp.StatusCode < 400 {
-								logger.LegacyPrintf("service.gateway", "Account %d: signature error retry succeeded (thinking downgraded)", account.ID)
+								logger.LegacyPrintf("service.gateway", "Account %d: thinking block retry succeeded (blocks downgraded)", account.ID)
 								resp = retryResp
 								break
 							}
@@ -6102,13 +6102,9 @@ func (s *GatewayService) isThinkingBlockSignatureError(respBody []byte) bool {
 		return false
 	}
 
-	// Log for debugging
-	logger.LegacyPrintf("service.gateway", "[SignatureCheck] Checking error message: %s", msg)
-
 	// 检测signature相关的错误（更宽松的匹配）
 	// 例如: "Invalid `signature` in `thinking` block", "***.signature" 等
 	if strings.Contains(msg, "signature") {
-		logger.LegacyPrintf("service.gateway", "[SignatureCheck] Detected signature error")
 		return true
 	}