Fix serializer for CreateElicitationResult

[tiginamaria]

Motivation and Context

Fixes: #233

The issue appeared due to fallback-style RequestResultPolymorphicSerializer implementation:

internal object ServerResultPolymorphicSerializer :
    JsonContentPolymorphicSerializer<ServerResult>(ServerResult::class) {
    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<ServerResult> =
        selectServerResultDeserializer(element)
            ?: EmptyRequestResult.serializer()
}

internal object ClientResultPolymorphicSerializer :
    JsonContentPolymorphicSerializer<ClientResult>(ClientResult::class) {
    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<ClientResult> =
        selectClientResultDeserializer(element)
            ?: EmptyRequestResult.serializer()
}

internal object RequestResultPolymorphicSerializer :
    JsonContentPolymorphicSerializer<RequestResult>(RequestResult::class) {
    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<RequestResult> =
        selectClientResultDeserializer(element)
            ?: selectServerResultDeserializer(element)
            ?: EmptyRequestResult.serializer()
}

1. CreateElicitationResult is a ClientResult with an action field https://modelcontextprotocol.io/specification/draft/client/elicitation
2. The selectClientResultDeserializer function was missing a case for the action field
3. Since it wasn't found in the client results, the system fell back to server result deserialization
4. The selectServerResultDeserializer found the content field and incorrectly mapped it to CallToolResult.serializer()

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[devcrocod]

lgtm!