boundary

Using plain string streaming, Anthropic provider.
Anthropic likes to have random ephemeral errors sometimes, e.g.:
```
reqwest::Error { kind: Request, url: "https://api.anthropic.com/v1/messages", source: hyper_util::client::legacy::Error(SendRequest, hyper::Error(Io, Custom { kind: Other, error: "bad MAC" })) }
```
In these cases, b.stream will RETRY the request, but will NOT yield the text chunks of the restarted stream. It will instead yield nothing (silent fail).

Would expect it to stream the chunks of the restarted stream.
Maybe I could work around this by disabling BAML's retries, but seems like this is a BAML bug?
The more ambiguous case would be if the first stream had streamed N chunks first (instead of 0) before failing - but I've never actually encountered that so that doesn't matter for me. If it fails its always immediate, and I would want it to pick up fresh from the restarted stream.

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