Anthropic model-list requests currently flatten request, transport,
response, and API failures into `anyhow` strings. Callers therefore
cannot distinguish an invalid API key from connectivity or provider
failures, even though Anthropic returns a structured error payload.

This changes `list_models` to return `AnthropicError`, maps each request
stage to its existing typed variant, and routes unsuccessful responses
through the same structured response handler used by completion
requests. The language model provider converts that error at its
existing `LanguageModelCompletionError` boundary. A regression test
verifies that an authentication response retains both its error category
and Anthropic's human-readable message.

Testing performed:

- `cargo fmt --check`
- `cargo nextest run -p anthropic -p language_models`
- `./script/clippy -p anthropic -p language_models`

Release Notes:

- Fixed Anthropic API key errors being reported as generic model-list
failures.