feat: add cost and cost_details to ChatGenerationTokenUsage schema

[untilhamza]

Summary

The OpenRouter API returns cost, is_byok, and cost_details in the usage object of chat completion responses, but the OpenAPI spec doesn't define these fields — so they're silently dropped during model_dump().

This PR adds the missing fields to ChatGenerationTokenUsage in in.openapi.yaml.

Fields added

Field	Type	Description
cost	number, nullable	Total generation cost in USD
is_byok	boolean, nullable	Whether the request used bring-your-own-key
cost_details.upstream_inference_cost	number, nullable	Total upstream inference cost
cost_details.upstream_inference_prompt_cost	number, nullable	Upstream cost for prompt tokens
cost_details.upstream_inference_completions_cost	number, nullable	Upstream cost for completion tokens

The problem

Raw HTTP response includes cost data:

{
  "usage": {
    "prompt_tokens": 158,
    "completion_tokens": 152,
    "total_tokens": 310,
    "cost": 8.41005e-05,
    "is_byok": false,
    "cost_details": {
      "upstream_inference_cost": 8.495e-05,
      "upstream_inference_prompt_cost": 8.95e-06,
      "upstream_inference_completions_cost": 7.6e-05
    }
  }
}

But after the SDK deserializes via model_dump(), cost and cost_details are gone:

from openrouter import OpenRouter

client = OpenRouter()
resp = client.chat.send(model="x-ai/grok-4.1-fast", messages=[...])
print(resp.usage.cost)  # AttributeError — field doesn't exist

Verification

I patched ChatGenerationTokenUsage locally to add these fields and confirmed cost data flows through correctly after model_dump().

Fixes #65

[untilhamza]

@mattapperson , @robert-j-y please take a look at this.