Refine to_openapi to project operation-level errors (with http_status)
onto /call and /subscribe responses via oneOf merge with protocol-level
errors, preserving HTTP_<status> prefix codes without collision. Fix
BTreeMap→serde_json::Map for Value::Object compatibility. Wire GET
/openapi.json route test. Apply cargo fmt across the crate.
Adds SharedHttpClient wrapping ArcSwap<ClientWithMiddleware> with a
RetryTransientMiddleware + inlined RetryAfterMiddleware stack.
HttpClientConfig covers pool, timeout, retry policy, and optional CA
bundle/client cert. reload() rebuilds and swaps via ArcSwap. No env-var
reads; credential injection is per-request, not at construction.
