We were sending the `speed` field set to `"standard"` for BYOK Anthropic
but without the corresponding beta header. leading the requests to fail
with "invalid request format to Anthropic's API: speed: Extra inputs are
not permitted".

This makes sure to attach the beta header whenever the `speed` parameter
is used.

Release Notes:

- Fixed "speed: Extra inputs are not permitted" errors for Opus 4.6 and
4.7 in the Anthropic API provider.