Serializes query maps to the API's bracket syntax, exactly as the Node SDK.

iex> Lettermint.Query.encode(%{page: %{size: 10}, filter: %{status: "verified"}, sort: ["-created_at"]})
"filter%5Bstatus%5D=verified&page%5Bsize%5D=10&sort=-created_at"

• nested maps (or keyword lists) become bracketed names: page[size];
• lists of values are joined with commas: sort=-created_at,domain;
• lists of maps are indexed: filter[tags][0][name]=a;
• booleans are 1 or 0; DateTime, NaiveDateTime and Date are ISO 8601;
• nil values and empty lists are left out.

Keys of a map are sent in sorted order; keyword lists keep their order. Names and values are form-encoded like URLSearchParams.