Partners HealthCare does not hold back in the response it plans to file today with the state's Health Policy Commission (HPC). The commission issued a report last month that marked a rare effort to crimp Partners' dominance in the Massachusetts market. The commission said that if Partners adds South Shore Hospital in Weymouth to its growing network, costs will increase around $23-26 million a year. Wrong, says Partners, in an 89-page rebuttal that includes dozens of letters and testimonials from South Shore area leaders who support the merger. The commission should withdraw its finding, concludes Partners, and not send the proposed merger to the state attorney general for further regulatory review.