Follow-up to the workspace-wide error chain preservation: removes the
`From<String>` impl on `PullRequestApiError`, the unused `SharedError::as_anyhow`,
and the test-only `DisplayContains`/`DisplayStringExt` traits that papered over
String errors. Call sites now build `anyhow!` errors directly and tests stringify
errors explicitly via `.to_string()`.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>