feat(internal): add ListOr[T any] type

This is a utility type that lets you decode a JSON T or list of T as a
single value. This will be used with Redis Sentinel config so that you
can specify multiple sentinel addresses.

Ref TecharoHQ/botstopper#24

Assisted-by: GLM 4.6 via Claude Code
Signed-off-by: Xe Iaso <me@xeiaso.net>

internal/listor.go

@@ -0,0 +1,39 @@
+package internal
+
+import (
+	"encoding/json"
+)
+
+// ListOr[T any] is a slice that can contain either a single T or multiple T values.
+// During JSON unmarshaling, it checks if the first character is '[' to determine
+// whether to treat the JSON as an array or a single value.
+type ListOr[T any] []T
+
+func (lo *ListOr[T]) UnmarshalJSON(data []byte) error {
+	if len(data) == 0 {
+		return nil
+	}
+
+	// Check if first non-whitespace character is '['
+	firstChar := data[0]
+	for i := 0; i < len(data); i++ {
+		if data[i] != ' ' && data[i] != '\t' && data[i] != '\n' && data[i] != '\r' {
+			firstChar = data[i]
+			break
+		}
+	}
+
+	if firstChar == '[' {
+		// It's an array, unmarshal directly
+		return json.Unmarshal(data, (*[]T)(lo))
+	} else {
+		// It's a single value, unmarshal as a single item in a slice
+		var single T
+		if err := json.Unmarshal(data, &single); err != nil {
+			return err
+		}
+		*lo = ListOr[T]{single}
+	}
+
+	return nil
+}