fix(gateway): strip empty text blocks from nested tool_result content

Empty text blocks inside tool_result.content were not being filtered,
causing upstream 400 errors: 'text content blocks must be non-empty'.

Changes:
- Add stripEmptyTextBlocksFromSlice helper for recursive content filtering
- FilterThinkingBlocksForRetry now recurses into tool_result nested content
- Add StripEmptyTextBlocks pre-filter on initial request path to avoid
  unnecessary 400+retry round-trips
- Add unit tests for nested empty text block scenarios

backend/internal/service/gateway_service.go

@@ -4119,6 +4119,9 @@ func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *A
 	// 调试日志：记录即将转发的账号信息
 	logger.LegacyPrintf("service.gateway", "[Forward] Using account: ID=%d Name=%s Platform=%s Type=%s TLSFingerprint=%v Proxy=%s",
 		account.ID, account.Name, account.Platform, account.Type, account.IsTLSFingerprintEnabled(), proxyURL)
+	// Pre-filter: strip empty text blocks (including nested in tool_result) to prevent upstream 400.
+	body = StripEmptyTextBlocks(body)
+
 	// 重试间复用同一请求体，避免每次 string(body) 产生额外分配。
 	setOpsUpstreamRequestBody(c, body)
 
@@ -4603,6 +4606,9 @@ func (s *GatewayService) forwardAnthropicAPIKeyPassthroughWithInput(
 	if c != nil {
 		c.Set("anthropic_passthrough", true)
 	}
+	// Pre-filter: strip empty text blocks (including nested in tool_result) to prevent upstream 400.
+	input.Body = StripEmptyTextBlocks(input.Body)
+
 	// 重试间复用同一请求体，避免每次 string(body) 产生额外分配。
 	setOpsUpstreamRequestBody(c, input.Body)
 
@@ -7877,6 +7883,9 @@ func (s *GatewayService) ForwardCountTokens(ctx context.Context, c *gin.Context,
 	body := parsed.Body
 	reqModel := parsed.Model
 
+	// Pre-filter: strip empty text blocks to prevent upstream 400.
+	body = StripEmptyTextBlocks(body)
+
 	isClaudeCode := isClaudeCodeRequest(ctx, c, parsed)
 	shouldMimicClaudeCode := account.IsOAuth() && !isClaudeCode