A Phase 1b/2, Randomized, Open-Label Study Investigating the Efficacy and Safety of LBL-007 Plus Tislelizumab in Combination With Bevacizumab Plus Fluoropyrimidine Versus Bevacizumab Plus Fluoropyrimidine as Maintenance Therapy in Patients With Unresectable or Metastatic Microsatellite Stable/Mismatch Repair Proficient Colorectal Cancer