Fix: Prevent double MethodChannel result in Carrot.setup

[1leary1]

Problem

The Android implementation of Carrot.setup() could call MethodChannel.Result multiple times in certain error scenarios (e.g. HTTP 400 or RxJava composite errors).

This leads to a Flutter runtime crash:
java.lang.IllegalStateException: Reply already submitted

What fixed

Introduced a thread-safe guard using AtomicBoolean to ensure that the result is returned only once per method call.

val replied = AtomicBoolean(false)

if (replied.compareAndSet(false, true)) {
    result.success(...)
}