Twenty-eight cats were rescued yesterday from a rundown shed in Hampden County and taken to a Jamaica Plain animal shelter. A middle-aged woman had been living off the money she made from breeding and selling the exotic purebred cats for almost a decade, but her small shed filled up and she could not take care of them anymore, leading to the intervention, said Christine Allenberg, the MSPCA enforcement officer who rescued the cats. The cats, which were in poor condition, were kept in several small cages in a shed that measured about 12-by-24 feet and was 7 or 8 feet tall, Allenberg said. The MSPCA would not disclose the name or location of the woman because she willingly surrendered the animals.