fix: recurse into []*struct with Deep struct-to-map decoding (#196)

[chiliec]

Fixes #196.

Problem

With DecoderConfig.Deep (or a ,deep tag), struct-to-map decoding recurses into []MyStruct and produces []map[string]any, but a []*MyStruct field is left as raw []*MyStruct in the output map. The two slice shapes decode inconsistently.

Root cause is in decodeMapFromStruct's reflect.Slice branch, which only recurses when the element Kind is reflect.Struct:

switch v.Type().Elem().Kind() {
case reflect.Struct:
    childType = reflect.TypeOf(map[string]any{})
default:
    childType = v.Type().Elem()   // []*T lands here → copied verbatim
}

For []*T the element Kind is reflect.Ptr, so it falls into default and the pointer elements are copied as-is.

Fix

Treat a slice whose element is a pointer-to-struct the same as a slice of structs, mapping both to []map[string]any. The existing d.decode call already dereferences the pointer elements, so no other change is needed.

Test

Extended the existing TestDecode_structArrayDeepMap with a ChildrenC []*SourceChild field alongside the current []SourceChild and *[]SourceChild cases. It fails before the change (children-c comes back as []*mapstructure.SourceChild) and passes after ([]map[string]any).

Verification (real runs, Go 1.24.6)

• RED: with the test but without the mapstructure.go change → FAIL: TestDecode_structArrayDeepMap (children-c = []*mapstructure.SourceChild{...}).
• GREEN: with the fix → PASS.
• Full suite as CI runs it: go test -race -shuffle=on ./... → ok github.com/go-viper/mapstructure/v2.
• gofmt -s, gofumpt, and go vet ./... all clean.

AI assistance disclosure

Prepared with an AI coding assistant; the root-cause analysis, fix, extended test, and the RED→GREEN + race/lint verification above were run against a real Go 1.24.6 toolchain, not generated.