SerialDescriptor.getElementIndex can throw

[ansman]

Describe the bug
The documentation states that SerialDescriptor.getElementIndex returns the index or UNKNOWN_NAME when there is no such element. But the map like descriptor will throw if you pass a non integer like descriptor.

To Reproduce

val serializer = MapSerializer(String.serializer(), String.serializer())
val descriptor = serializer.descriptor
descriptor.getElementIndex("key") // Throws

Expected behavior
There is no documentation that this function will ever throw and it would be more suitable if it returned UNKNOWN_NAME.

Environment

• Kotlin version: 2.3.10
• Library version: 1.10.0
• Kotlin platforms: JVM