 Platform Study to Evaluate Mirdametinib as a Combination Therapy with Fulvestrant in Patients with ER Metastatic Breast Cancer and as a Monotherapy in Patients with Advanced Solid Tumors Harboring Activating Mutations in MEK1 or MEK2 Platform Study to Evaluate Mirdametinib as a Combination Therapy with Fulvestrant in Patients with ER Metastatic Breast Cancer and as a Monotherapy in Patients with Advanced Solid Tumors Harboring Activating Mutations in MEK1 or MEK2 