Implements call_error_to_http_status, call_error_to_http_status_with_identity,
and call_error_to_http_response in src/gateway/error.rs. Five protocol codes
map to fixed statuses (404/422/504/500 + 401/403 split for FORBIDDEN).
HTTP_<status>-prefixed operation-level codes parse status from prefix. Unknown
operation-level codes default to 500. Retry-After header for retryable 503/429.
21 unit tests.
# Conflicts:
# crates/alknet-http/src/gateway/mod.rs