fix(aux-client): preserve /anthropic for MiniMax M-series auxiliary routes

[gsurenull]

Summary

Fixes #17387.

Auxiliary tasks (title generation, compression, session_search, vision analysis, flush_memories, ...) for the built-in minimax, minimax-oauth, and minimax-cn providers were 404'ing because _to_openai_base_url rewrote https://api.minimaxi.com/anthropic -> https://api.minimaxi.com/v1 before handing the URL to the OpenAI SDK. M-series models on api.minimaxi.com and api.minimax.io do not expose a working /v1/chat/completions surface — they only speak Anthropic Messages on /anthropic/v1/messages.

Fix

Preserve the original /anthropic URL for MiniMax hosts and rely on the existing _maybe_wrap_anthropic chokepoint to detect the Anthropic-Messages shape via _endpoint_speaks_anthropic_messages and route through the Anthropic SDK. This mirrors the named-custom api_mode=anthropic_messages path added in #15059 but covers the built-in provider profile path that was missed by that fix.

Scope

• MiniMax hosts (api.minimaxi.com, api.minimax.io): preserved (new behaviour)
• ZAI (open.bigmodel.cn): continues to rewrite to /api/paas/v4 (regression guard added)
• Generic /anthropic URLs (LiteLLM proxies, etc.): continues to rewrite to /v1 (regression guard added)

Tests

• 11/11 new assertions pass on the branch
• 201 adjacent tests (test_auxiliary_named_custom_providers, test_auxiliary_client) pass with no regressions
• New TestEndToEndAnthropicRouting covers the full chain: preserved URL is still recognised by _endpoint_speaks_anthropic_messages so calls land on Anthropic SDK wire, not OpenAI wire

Diff stat

agent/auxiliary_client.py                 | 30 ++++++++++----
tests/agent/test_minimax_auxiliary_url.py | 66 +++++++++++++++++++++++--------
2 files changed, 72 insertions(+), 24 deletions(-)