`cost` and `cost_details` missing from `ChatGenerationTokenUsage` schema

[untilhamza]

Summary

The OpenRouter API returns cost and cost_details in the usage object of chat completion responses, but the SDK's ChatGenerationTokenUsage model doesn't define these fields, so they get silently dropped during deserialization.

API response (raw HTTP)

{
  "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
    },
    "completion_tokens_details": { "..." : "..." },
    "prompt_tokens_details": { "..." : "..." }
  }
}

SDK result after model_dump()

{
    "completion_tokens": 152,
    "prompt_tokens": 158,
    "total_tokens": 310,
    "completion_tokens_details": { ... },
    "prompt_tokens_details": { ... }
    # cost and cost_details are gone
}

Root cause

ChatGenerationTokenUsage in src/openrouter/components/chatgenerationtokenusage.py only defines completion_tokens, prompt_tokens, total_tokens, completion_tokens_details, and prompt_tokens_details. The cost and cost_details fields aren't in the OpenAPI spec, so they're not generated.

Expected fix

Add to the OpenAPI spec (ChatGenerationTokenUsage schema):

cost:
  type: number
  nullable: true
  description: "Total cost of the generation in USD"
cost_details:
  type: object
  nullable: true
  properties:
    upstream_inference_cost:
      type: number
      nullable: true
    upstream_inference_prompt_cost:
      type: number
      nullable: true
    upstream_inference_completions_cost:
      type: number
      nullable: true
  description: "Breakdown of generation cost"

Verified locally

I patched the installed SDK to add cost: OptionalNullable[float] and cost_details: OptionalNullable[CostDetails] to ChatGenerationTokenUsage and confirmed the data flows through correctly after model_dump().

Impact

This blocks downstream packages (e.g. langchain-openrouter) from surfacing cost data, even though the langchain integration already has code to read cost from the usage dict. The SDK silently drops it before langchain ever sees it.

[untilhamza]

PR for this fix; #66