Experimental: Dose Escalation, RP2D Confirmatory, and Expansion (Phase 1/2 Single Arm)

Dose Escalation Cohort: Patients receive a single infusion of ECT204 T cells at one of four predefined dose levels on Day 0 after conditioning. Conditioning consists of fludarabine (Flu) and cyclophosphamide (Cy).~RP2D Confirmatory Cohort: Patients receive ECT204 T cells at the RP2D on Day 0 and may receive a second infusion approximately one month later. This cohort uses the same Flu/Cy conditioning as the dose-escalation cohort; no conditioning is given before the second ECT204 infusion.~Expansion Cohort: Patients receive multiple ECT204 infusions at the RP2D (initial on Day 0, planned second on Day 31, and optional third or later doses). The third infusion may be administered no earlier than Day 60, and each subsequent infusion must be separated by at least 30 days. Patients receive Flu/Cy/regorafenib before the first infusion and regorafenib alone before the second and subsequent infusions.