fix(aux-client): honor api_mode: anthropic_messages for named custom providers (#15059)

Auxiliary tasks (session_search, flush_memories, approvals, compression,
vision, etc.) that route to a named custom provider declared under
config.yaml 'providers:' with 'api_mode: anthropic_messages' were
silently building a plain OpenAI client and POSTing to
{base_url}/chat/completions, which returns 404 on Anthropic-compatible
gateways that only expose /v1/messages.

Two gaps caused this:

1. hermes_cli/runtime_provider.py::_get_named_custom_provider — the
   providers-dict branch (new-style) returned only name/base_url/api_key/
   model and dropped api_mode. The legacy custom_providers-list branch
   already propagated it correctly. The dict branch now parses and
   returns api_mode via _parse_api_mode() in both match paths.

2. agent/auxiliary_client.py::resolve_provider_client — the named
   custom provider block at ~L1740 ignored custom_entry['api_mode']
   and unconditionally built an OpenAI client (only wrapping for
   Codex/Responses). It now mirrors _try_custom_endpoint()'s three-way
   dispatch: anthropic_messages → AnthropicAuxiliaryClient (async wrapped
   in AsyncAnthropicAuxiliaryClient), codex_responses → CodexAuxiliaryClient,
   otherwise plain OpenAI. An explicit task-level api_mode override
   still wins over the provider entry's declared api_mode.

Fixes #15033

Tests: tests/agent/test_auxiliary_named_custom_providers.py gains a
TestProvidersDictApiModeAnthropicMessages class covering

  - providers-dict preserves valid api_mode
  - invalid api_mode values are dropped
  - missing api_mode leaves the entry unchanged (no regression)
  - resolve_provider_client returns (Async)AnthropicAuxiliaryClient for
    api_mode=anthropic_messages
  - full chain via get_text_auxiliary_client / get_async_text_auxiliary_client
    with an auxiliary.<task> override
  - providers without api_mode still use the OpenAI-wire path

agent/auxiliary_client.py

@@ -1736,7 +1736,7 @@ def resolve_provider_client(
                        "but no endpoint credentials found")
         return None, None
 
-    # ── Named custom providers (config.yaml custom_providers list) ───
+    # ── Named custom providers (config.yaml providers dict / custom_providers list) ───
     try:
         from hermes_cli.runtime_provider import _get_named_custom_provider
         custom_entry = _get_named_custom_provider(provider)
@@ -1747,16 +1747,51 @@ def resolve_provider_client(
             if not custom_key and custom_key_env:
                 custom_key = os.getenv(custom_key_env, "").strip()
             custom_key = custom_key or "no-key-required"
+            # An explicit per-task api_mode override (from _resolve_task_provider_model)
+            # wins; otherwise fall back to what the provider entry declared.
+            entry_api_mode = (api_mode or custom_entry.get("api_mode") or "").strip()
             if custom_base:
                 final_model = _normalize_resolved_model(
                     model or custom_entry.get("model") or _read_main_model() or "gpt-4o-mini",
                     provider,
                 )
-                client = OpenAI(api_key=custom_key, base_url=custom_base)
-                client = _wrap_if_needed(client, final_model, custom_base)
                 logger.debug(
-                    "resolve_provider_client: named custom provider %r (%s)",
-                    provider, final_model)
+                    "resolve_provider_client: named custom provider %r (%s, api_mode=%s)",
+                    provider, final_model, entry_api_mode or "chat_completions")
+                # anthropic_messages: route through the Anthropic Messages API
+                # via AnthropicAuxiliaryClient. Mirrors the anonymous-custom
+                # branch in _try_custom_endpoint(). See #15033.
+                if entry_api_mode == "anthropic_messages":
+                    try:
+                        from agent.anthropic_adapter import build_anthropic_client
+                        real_client = build_anthropic_client(custom_key, custom_base)
+                    except ImportError:
+                        logger.warning(
+                            "Named custom provider %r declares api_mode="
+                            "anthropic_messages but the anthropic SDK is not "
+                            "installed — falling back to OpenAI-wire.",
+                            provider,
+                        )
+                        client = OpenAI(api_key=custom_key, base_url=custom_base)
+                        return (_to_async_client(client, final_model) if async_mode
+                                else (client, final_model))
+                    sync_anthropic = AnthropicAuxiliaryClient(
+                        real_client, final_model, custom_key, custom_base, is_oauth=False,
+                    )
+                    if async_mode:
+                        return AsyncAnthropicAuxiliaryClient(sync_anthropic), final_model
+                    return sync_anthropic, final_model
+                client = OpenAI(api_key=custom_key, base_url=custom_base)
+                # codex_responses or inherited auto-detect (via _wrap_if_needed).
+                # _wrap_if_needed reads the closed-over `api_mode` (the task-level
+                # override). Named-provider entry api_mode=codex_responses also
+                # flows through here.
+                if entry_api_mode == "codex_responses" and not isinstance(
+                    client, CodexAuxiliaryClient
+                ):
+                    client = CodexAuxiliaryClient(client, final_model)
+                else:
+                    client = _wrap_if_needed(client, final_model, custom_base)
                 return (_to_async_client(client, final_model) if async_mode
                         else (client, final_model))
             logger.warning(