fix(gconv): route map-slice elements through MapToMap in Structs (#4787)

[waterWang]

Fix: Result.Structs / Model.Scan into *[]map[string]interface{} silently returns empty maps

Description

When db.Model().Scan(&rows) or Result.Structs(&rows) is called with *[]map[string]interface{}, it returns a slice of the correct length but every element is an empty map, with no error returned. Switching to a typed struct slice, or using Result.List() / gconv.Scan(), works correctly.

Root Cause

In converter.Structs (converter_structs.go), the destination element type is checked:

if itemTypeKind == reflect.Pointer {
    // Pointer element — handles correctly
} else {
    // "Struct element" — taken for reflect.Map too
    ...
    c.Struct(paramsList[i], tempReflectValue, ...)  // expects a struct!
}

c.Struct calls GetCachedStructInfo() which returns nil for non-struct types (like reflect.Map), causing a silent no-op. The pre-allocated slice (correct length) is returned with each map left at its zero value.

By contrast, gconv.Scan → converter.Scan → doScanForComplicatedTypes does have the map-slice branch (sliceElemKind == reflect.Map → MapToMaps), which is why gconv.Scan and Result.List() work correctly.

Fix

Added a reflect.Map branch in converter.Structs that routes map-typed elements through c.MapToMap instead of c.Struct:

} else if itemTypeKind == reflect.Map {
    // Map element — use MapToMap instead of Struct.
    for i := 0; i < len(paramsList); i++ {
        ...
        c.MapToMap(paramsList[i], tempReflectValue, nil, MapOption{...})
        ...
    }
} else {
    // Struct element (unchanged)
    ...
}

This matches the existing behavior of gconv.Scan's doScanForComplicatedTypes method, which already handles map-slice conversion via MapToMaps.

Testing

The reproduction case from the issue now produces correct output:

res := gdb.Result{
    gdb.Record{"regionId": gvar.New(1)},
    gdb.Record{"regionId": gvar.New(2)},
    gdb.Record{"regionId": gvar.New(3)},
}
var viaStructs []map[string]interface{}
_ = res.Structs(&viaStructs)
// Before: len=3, first=map[]   (empty maps, no error)
// After:  len=3, first=map[regionId:1]  (correct data)

Fixes #4787