opencode_transport() re-derived the wire for every OpenCode-family custom
entry, while hermes_cli/runtime_provider_custom.py::_resolve_named_custom_runtime
only re-derives when the entry declares no api_mode. Aux now mirrors that gate,
so an entry pinned to chat_completions stays a plain chat client on both paths.
Test grid trimmed to [None, one wrong mode]; the bridge fixture drops its
api_mode (the override no longer applies to it) and a pinned entry asserts the
honoured mode instead.
1a2bf64aa7