This is a phase I, open-label, single-arm, single-center, dose-escalation study to evaluate the safety, tolerability, and preliminary anti-tumor efficacy of PAN-NeoVax, an artificial intelligence (AI)-designed pan-cancer neoantigen mRNA vaccine delivered via lipid nanoparticles (LNP), administered by intratumoral injection in patients with advanced or metastatic solid tumors who have failed second-line therapy. The study employs a classical "3+3" dose-escalation design with three dose levels (25 μg, 50 μg, and 100 μg mRNA). Each subject will receive 5 doses of basic immunization (first 4 doses at weekly intervals, fifth dose at 1 month after the fourth dose).