Phase I (Single Administration, Randomized, Double-blind, Dose Escalation)Primary Objective: To evaluate the safety and tolerability of Human Umbilical Cord Mesenchymal Stem Cell Injection in patients with decompensated hepatitis B cirrhosis, and to determine the safe clinical dose.Secondary Objective: To evaluate the preliminary efficacy of Human Umbilical Cord Mesenchymal Stem Cell Injection in patients with decompensated hepatitis B cirrhosis, providing a basis for the design of subsequent clinical trial protocols.Exploratory Objective: To evaluate the pharmacokinetic characteristics and immunogenicity of Human Umbilical Cord Mesenchymal Stem Cell Injection in patients with decompensated hepatitis B cirrhosis.Phase II (Multiple Administrations, Randomized, Double-blind, Dose Expansion)Primary Objective: To evaluate the preliminary efficacy of Human Umbilical Cord Mesenchymal Stem Cell Injection in patients with decompensated hepatitis B cirrhosis.Secondary Objective: To evaluate the safety of Human Umbilical Cord Mesenchymal Stem Cell Injection in patients with decompensated hepatitis B cirrhosis.Exploratory Objective: To evaluate the pharmacokinetic characteristics and immunogenicity of Human Umbilical Cord Mesenchymal Stem Cell Injection in patients with decompensated hepatitis B cirrhosis.