Minor refactor that I'm extracting from a branch because it can stand
alone.

- Now we no longer spawn an executor for `report_anthropic_event` if
it's just going to immediately fail due to API key being missing
- `report_anthropic_event` now takes a `String` API key instead of
`Option<String>` and the error reporting if the key is missing has been
moved to the caller.
- `report_anthropic_event` is longer coupled to `AnthropicError`,
because all it ever did was generate an `AnthropicEvent::Other`, which
in turn was then only used for `log_err` - so, can just be an
`anyhow::Result`.

Release Notes:

- N/A