A 3-year-old boy is brought to the ophthalmology clinic by his mother with a complaint of inward deviation of the left eye noticed since 6 months of age. On examination, the child has a visual acuity of 6/6 in both eyes. The cover test shows that when the right eye is covered, the left eye moves outward to fixate, and when the left eye is covered, the right eye moves inward. The alternate cover test confirms a 25 prism diopter esotropia. Cycloplegic refraction shows +2.00 D sphere in both eyes. The child has no history of prematurity, developmental delay, or neurological symptoms. What is the most likely diagnosis?