Problem

`anvil ci job-view ` against a completed job currently fails with the entire HTML error page pasted into the error string:

``` $ anvil ci job-view 7a1731da43 CI Job 7a1731da43 Name clippy Status passed …

Error: API error (406):

The server is returning a 406 with an HTML body. The CLI dumps the body verbatim because `Client::get_sse_stream` (and friends) just stuff `resp.text()` into `ApiError::Api.message` regardless of `Content-Type`.

There is a parallel server-side bug (the 406 itself, and HTML-instead-of-JSON content negotiation) — that is tracked separately on `fangorn/anvil`. This ticket is the CLI half.

Acceptance criteria

• When the API returns a non-2xx response with `Content-Type: text/html`, the CLI’s `ApiError::Api.message` contains a short, human-readable message — not the raw HTML body. Format: `server returned (HTML response from )`.
• When the API returns a non-2xx response with JSON, behavior is unchanged (we still surface the JSON body).
• Unit tests cover happy and unhappy paths for the response-handling helpers in `src/client.rs`:
  • happy: 2xx JSON parses into the expected type
  • unhappy: non-2xx with JSON body → `ApiError::Api` containing the JSON string
  • unhappy: non-2xx with HTML body → `ApiError::Api` with a short summary, NOT the HTML
  • unhappy: SSE stream variant of the same
• Tests run via `cargo test` and pass in CI.

Scope notes

• Touch `get_sse_stream`, `handle_response`, `get_raw`, `delete_empty`, `post_empty`, `upload_file`, `download_file` — anywhere we currently take `resp.text()` straight into `ApiError`.
• No new dependencies if avoidable; `wiremock` or `mockito` is acceptable as a dev-dependency for the tests.