Bubbles up rate limit information so that we can retry after a certain
duration if needed higher up in the stack.
Also caps the number of concurrent evals running at once to also help.
Release Notes:
- N/A
This PR updates the Agent to extract the usage information from the
response headers, if they are present.
For now we just log the information, but we'll be using this soon to
populate some UI.
Release Notes:
- N/A
We also eliminate the `completion` crate and moved its logic into
`LanguageModelRegistry`.
Release Notes:
- N/A
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Co-authored-by: Nathan <nathan@zed.dev>
