The SDK uses request parameters in camel case instead of snake case

[alexkojin]

first_message param rejected, only works with camelCase

Hey folks — ran into an odd one when creating an assistant using the Ruby SDK.

If I use first_message (snake_case), I get this error:

#<Faraday::BadRequestError response={status:400, body:
  {"message":["property first_message should not exist"],
   "error":"Bad Request","statusCode":400}}>

But if I switch to firstMessage (camelCase), everything works fine.

Here’s a quick repro:

require 'vapi_server_sdk'

client = Vapi::Client.new(token: Rails.application.credentials.vapi.api_key)

assistant = client.assistants.create(request: {
  name: "test",
  first_message: "Hi there, this is Alex from. How can I help you today?",
})

Looks like the SDK expects Ruby-style snake_case, but the API only accepts camelCase.
Probably a serialization mismatch somewhere in [lib/vapi_server_sdk/assistants/client.rb](https://github.com/VapiAI/server-sdk-ruby/blob/main/lib/vapi_server_sdk/assistants/client.rb#L599).

SDK version: 0.9.1
Ruby: 3.4
Rails: 7.2

Would be great if the SDK handled the conversion automatically so we can stick to idiomatic Ruby naming or change code documentation.