Add `cacheRead` and `cacheWrite` to workers-ai usage

[Specy]

Looking at the usage code it seems like cacheRead, cacheWrite and noCache are not mapped correctly and are always undefined.

I use the usage to calculate pricing of a request, cacheRead/Write are very important to understand the actual costs of a call.

[samscott89]

The usage metrics come back as:

{
   ...
  "usage": {
    "prompt_tokens": 1695,
    "completion_tokens": 76,
    "total_tokens": 1771,
    "prompt_tokens_details": {
      "cached_tokens": 1694
    }
  }

So we'll need to pull out prompt_tokens_details.cached_token

[Specy]

Adding a debugger to usage in the mapWorkersAiUsage i can confirm the schema, here is an example i got:

{
  response: '',
  usage: {
    prompt_tokens: 6377,
    completion_tokens: 349,
    total_tokens: 6726,
    prompt_tokens_details: { cached_tokens: 2861 }
  }
}

[threepointone]

thanks for the pr, released