fix(aux): honor explicit base_url for named providers; preserve /anthropic

[wmagev]

Summary

Three related config-inheritance bugs in agent/auxiliary_client.py that
surface when the auxiliary client (compression, title generation, vision,
session search, etc.) tries to reuse the user's main provider config:

1. _resolve_auto only forwarded base_url for custom / custom:*
   providers (Auxiliary tasks (compression, title generation) ignore custom base_url for named providers #16719). With provider: zai + base_url: <override>, the
   override was silently dropped and auxiliary traffic went to z.ai's
   default endpoint. Now forwarded as explicit_base_url /
   explicit_api_key for any named provider.

2. PROVIDER_REGISTRY branch ignored explicit_base_url / explicit_api_key.
   Even after fix (1), the named-provider branch unconditionally pulled
   base_url from creds["base_url"] or pconfig.inference_base_url. Now
   prefers the forwarded explicit values.

3. custom branch unconditionally rewrote /anthropic → /v1 (Bug: auxiliary tasks fail with HTTP 404 when using anthropic_messages custom endpoint — URL rewrite strips /anthropic/ path #17086).
   _to_openai_base_url() is the right normalization for OpenAI-wire
   gateways, but for api_mode: anthropic_messages (or any URL the
   _endpoint_speaks_anthropic_messages() heuristic recognizes) the
   rewrite breaks the AnthropicAuxiliaryClient — requests land on
   /v1/chat/completions instead of /anthropic/v1/messages → 404 on
   every aux task. The same guard already existed in
   _try_custom_endpoint() and the named-custom-provider branch; this PR
   extends it to the explicit_base_url custom branch and the
   PROVIDER_REGISTRY branch.

Out of scope (mentioned in the original issues but separately resolved):

• github-copilot slug normalization ([Bug]: auxiliary runtime does not normalize github-copilot provider slugs #15017) was already addressed in
  commit 2e2de12.
• title_generator.py / context_compressor.py already pass main_runtime
  to call_llm(); the original issue's claim about missing main_runtime
  is stale.

Test plan

• tests/agent/test_auxiliary_named_custom_providers.py — 31 passed
  (3 new test classes, 5 new tests)
• tests/agent/test_auxiliary_client.py — 194 sync tests pass; 5
  pre-existing async failures unrelated (require pytest-asyncio)
• tests/agent/test_auxiliary_client_anthropic_custom.py — passes
  (existing _try_custom_endpoint anthropic_messages coverage)
• tests/agent/test_auxiliary_main_first.py,
  test_auxiliary_transport_autodetect.py,
  test_minimax_auxiliary_url.py,
  test_auxiliary_config_bridge.py — all pass
• tests/agent/test_context_compressor.py — 65 passed (no compression
  regressions)
• Manual: configure provider: zai + base_url: https://custom-zai.example.com/v1 and verify compression hits the
  custom URL (logs show Auxiliary auto-detect: using main provider zai
  and the request goes to the override).
• Manual: configure custom Anthropic-compat proxy with api_mode: anthropic_messages + base_url: http://localhost:6655/anthropic/
  and verify auxiliary tasks no longer 404.

Closes: #16719, #17086

[alt-glitch]

Related to #16256 / #16254 — overlapping fixes for auxiliary_client.py explicit_base_url and api_mode handling. This PR appears to be a superset that addresses all three bugs (base_url forwarding, PROVIDER_REGISTRY ignoring explicit values, /anthropic rewrite).

[teknium1]

Closing — the /anthropic URL rewrite portion is now fixed by #17467 (merged as 4e296dc) via a more localized approach (preserve raw URL for transport detection). Your other two fixes — _resolve_auto forwarding base_url for named providers (#16719) and the PROVIDER_REGISTRY branch honoring explicit_base_url/explicit_api_key — are still valuable and not covered by #17467. Please re-scope to just those two fixes in a fresh PR and I'll review.