feat(migration): add support for altering column nullability

* Implement ExecuteAlterColumnNullability function
* Create alter_column_nullability template
* Add test for altering column nullability behavior

pkg/writers/pgsql/migration_writer.go

@@ -497,6 +497,29 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 				}
 				scripts = append(scripts, script)
 			}
+
+			// Check nullability changes
+			if modelCol.NotNull != currentCol.NotNull {
+				sql, err := w.executor.ExecuteAlterColumnNullability(AlterColumnNullabilityData{
+					SchemaName: schema.Name,
+					TableName:  modelTable.Name,
+					ColumnName: modelCol.Name,
+					NotNull:    modelCol.NotNull,
+				})
+				if err != nil {
+					return nil, err
+				}
+
+				script := MigrationScript{
+					ObjectName: fmt.Sprintf("%s.%s.%s", schema.Name, modelTable.Name, modelCol.Name),
+					ObjectType: "alter column nullability",
+					Schema:     schema.Name,
+					Priority:   145,
+					Sequence:   len(scripts),
+					Body:       sql,
+				}
+				scripts = append(scripts, script)
+			}
 		}
 	}