Unknown hosts (e.g. gateway.example.com) no longer get /anthropic→/v1;
use a real dual-surface MiniMax base for the rewrite assertions and add
a case proving Anthropic-only gateways keep their path on the OpenAI wire.
The custom + explicit_base_url branch of resolve_provider_client()
unconditionally rewrote a trailing /anthropic to /v1 via
_to_openai_base_url(), even when api_mode was anthropic_messages. The
Anthropic wrapper then never saw the real /anthropic path, so auxiliary
tasks (title generation, compression, vision, web_extract,
session_search) hit .../v1/chat/completions on a Messages-only endpoint
and failed.
Guard the wrap base on api_mode: for anthropic_messages, pass the raw
/anthropic base to _wrap_if_needed (which builds the Anthropic wrapper),
while the plain OpenAI client keeps the /v1-rewritten base so the
OpenAI-wire fallback (used when the anthropic SDK is unavailable) never
lands on /anthropic/chat/completions.
Refs #16254
Teknium