Neoadjuvant Tislelizumab + LM-302 + S-1 or Tislelizumab + SOX for CLDN18.2-Positive Gastric/GEJ Adenocarcinoma

Inclusion Criteria: * Patients voluntarily participate in this study and sign the informed consent form; * Age ≥ 18 years; * Histopathologically confirmed gastric adenocarcinoma or gastroesophageal junction adenocarcinoma; * HER2 negative; * Determined by contrast-enhanced CT and laparoscopy to have radically resectable disease with clinical stage T3-4 N+ M0 (according to the AJCC 8th edition); * Claudin 18.2 positive (≥25% of tumor cells showing moderate-to-strong membrane staining); * No prior receipt of other targeted therapies against claudin 18.2; * ECOG performance status 0-1; * Life expectancy ≥ 12 months; * Adequate major organ function. Exclusion Criteria: * Known HER2-positive gastric cancer； * Gastroesophageal junction (EGJ) cancer involving the proximal stomach with the tumor center located ≤2 cm from the EGJ； * Peritoneal metastasis, positive peritoneal cytology (CY1P0), or retroperitoneal lymph node metastasis (No. 16a2/b1) or other distant metastases; * Presence of unresectable factors, including unresectability due to tumor characteristics, surgical contraindications, or patient refusal of surgery; * Prior or concurrent other malignancy, except for cured basal cell carcinoma of the skin, carcinoma in situ of the cervix, and carcinoma in situ of the breast; * Presence of any of the following cardiac clinical symptoms or diseases: * New York Heart Association (NYHA) class ≥2 heart failure or left ventricular ejection fraction (LVEF) \<50% on color Doppler …