New: Detailed Cost Information in API Responses

API response JSON now includes detailed cost information for each request via a new cost object in the usage field.

"usage": {
  "prompt_tokens": 8,
  "completion_tokens": 439,
  "total_tokens": 447,
  "search_context_size": "low",
  "cost": {
    "input_tokens_cost": 2.4e-05,
    "output_tokens_cost": 0.006585,
    "request_cost": 0.006,
    "total_cost": 0.012609
  }
}

Includes: input_tokens_cost, output_tokens_cost, request_cost, total_cost.