fix: strip empty text blocks in retry filter and fix error pattern matching

Empty text blocks ({"type":"text","text":""}) cause Anthropic upstream to
return 400: "text content blocks must be non-empty". This was not caught
by the existing error detection pattern in isThinkingBlockSignatureError,
nor handled by FilterThinkingBlocksForRetry.

- Add empty text block stripping to FilterThinkingBlocksForRetry
- Fix isThinkingBlockSignatureError to match new Anthropic error format
- Add fast-path byte patterns to avoid unnecessary JSON parsing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/gateway_service.go

@@ -6067,9 +6067,11 @@ func (s *GatewayService) isThinkingBlockSignatureError(respBody []byte) bool {
 		return true
 	}
 
-	// 检测空消息内容错误（可能是过滤 thinking blocks 后导致的）
+	// 检测空消息内容错误（可能是过滤 thinking blocks 后导致的，或客户端发送了空 text block）
 	// 例如: "all messages must have non-empty content"
-	if strings.Contains(msg, "non-empty content") || strings.Contains(msg, "empty content") {
+	//       "messages: text content blocks must be non-empty"
+	if strings.Contains(msg, "non-empty content") || strings.Contains(msg, "empty content") ||
+		strings.Contains(msg, "must be non-empty") {
 		logger.LegacyPrintf("service.gateway", "[SignatureCheck] Detected empty content error")
 		return true
 	}