KotlinXSerializationMapper decodes lists to be size 1 always

[Niro]

Current implementation relies on kotlinx.serialization.descriptors.SerialDescriptor#getElementsCount. According to documentation it will be always 1.

    /**
     * The number of elements this descriptor describes, besides from the class itself.
     * [elementsCount] describes the number of **semantic** elements, not the number
     * of actual fields/properties in the serialized form, even though they frequently match.
     *
     * For example, for the following class
     * `class Complex(val real: Long, val imaginary: Long)` the corresponding descriptor
     * and the serialized form both have two elements, while for `class IntList : ArrayList<Int>()`
     * the corresponding descriptor has a single element (`IntDescriptor`, the type of list element),
     * but from zero up to `Int.MAX_VALUE` values in the serialized form.
     */
    @ExperimentalSerializationApi
    public val elementsCount: Int

As a result when Decoder tries to decode list field it gives up after first value. From screenshot below you can see that from underlying storage 16 elements was read but with current implementation it will contain only one element after decoding document to Kotlin data class.

[Niro]

Opened PR with a fix and test enhancement for this case