anthropic: update streaming usage metadata (#31158)
Anthropic updated how they report token counts during streaming today.
See changes to `MessageDeltaUsage` in [this
commit](https://github.com/anthropics/anthropic-sdk-python/commit/2da00f26c5f0bbf4f4039caec3c0500a2c2156bd#diff-1a396eba0cd9cd8952dcdb58049d3b13f6b7768ead1411888d66e28211f7bfc5).
It's clean and simple to grab these fields from the final
`message_delta` event. However, some of them are typed as Optional, and
language
[here](https://github.com/anthropics/anthropic-sdk-python/blob/e42451ab3f14443c0ed8076b17032a6d8986b6bf/src/anthropic/lib/streaming/_messages.py#L462)
suggests they may not always be present. So here we take the required
field from the `message_delta` event as we were doing previously, and
ignore the rest.
