fix(fallback): let custom_providers shadow built-in aliases

When a user defines `custom_providers: [{name: kimi, ...}]` and references
`provider: kimi` from fallback_model or the main config, the built-in alias
rewriting (`kimi` → `kimi-coding`) was hijacking the request before the
named-custom lookup ran.  `_get_named_custom_provider` also refused to
return a match when the raw name resolved to any built-in (including aliases),
so the custom endpoint was unreachable.

Fix at both layers of the resolution chain so every caller benefits, not
just `_try_activate_fallback`:

- hermes_cli/runtime_provider.py: narrow `_get_named_custom_provider`'s
  built-in-wins guard to canonical provider names only.  An alias like
  `kimi` that resolves to a different canonical (`kimi-coding`) no longer
  blocks the custom lookup; a canonical name like `nous` still does.

- agent/auxiliary_client.py: in `resolve_provider_client`, try the named-
  custom lookup with the original (pre-alias-normalization) name before the
  alias-normalized one, so aliased requests reach the user's custom entry.
  Also honour `explicit_base_url` and `explicit_api_key` in the API-key
  provider branch so callers that pass explicit hints (e.g. fallback
  activation) can override the registered defaults.

Tests added for:
- custom `kimi` shadowing built-in alias (regression for #15743)
- custom `nous` NOT shadowing canonical built-in (behaviour preserved)
- bare `kimi` without any custom entry still routing to built-in
- explicit base_url/api_key override on the API-key provider branch

Original PR #17827 by @Feranmi10 identified the same bug class and
implemented a narrower fix in `_try_activate_fallback`; this reshapes the
fix to live in the shared resolution layer so all callers benefit.

Fixes #15743
Co-authored-by: Feranmi10 <89228157+Feranmi10@users.noreply.github.com>

agent/auxiliary_client.py

@@ -1977,6 +1977,12 @@ def resolve_provider_client(
         (client, resolved_model) or (None, None) if auth is unavailable.
     """
     _validate_proxy_env_urls()
+    # Preserve the original provider name before alias normalization so a
+    # user-declared ``custom_providers`` entry whose name coincidentally
+    # matches a built-in alias (e.g. user names their custom provider "kimi"
+    # which aliases to "kimi-coding") is still reachable via the named-custom
+    # branch below.
+    original_provider = (provider or "").strip().lower()
     # Normalise aliases
     provider = _normalize_aux_provider(provider)
 
@@ -2163,7 +2169,18 @@ def resolve_provider_client(
     # ── 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)
+        # When the raw requested name is an alias (``kimi`` → ``kimi-coding``)
+        # and the user defined a ``custom_providers`` entry under that alias
+        # name, the custom entry is the intended target — the built-in alias
+        # rewriting would otherwise hijack the request.  Only preferred when
+        # the raw name is an alias (not a canonical provider name) so custom
+        # entries that coincidentally match a canonical provider (e.g. ``nous``)
+        # still defer to the built-in per `_get_named_custom_provider`'s guard.
+        custom_entry = None
+        if original_provider and original_provider != provider:
+            custom_entry = _get_named_custom_provider(original_provider)
+        if custom_entry is None:
+            custom_entry = _get_named_custom_provider(provider)
         if custom_entry:
             custom_base = custom_entry.get("base_url", "").strip()
             custom_key = custom_entry.get("api_key", "").strip()
@@ -2273,6 +2290,12 @@ def resolve_provider_client(
 
         creds = resolve_api_key_provider_credentials(provider)
         api_key = str(creds.get("api_key", "")).strip()
+        # Honour an explicit api_key override (e.g. from a fallback_model entry
+        # or a custom_providers entry) so callers that pass an explicit
+        # credential can authenticate against endpoints where no built-in
+        # credential is registered for this provider alias.
+        if explicit_api_key:
+            api_key = explicit_api_key.strip() or api_key
         if not api_key:
             tried_sources = list(pconfig.api_key_env_vars)
             if provider == "copilot":
@@ -2284,6 +2307,11 @@ def resolve_provider_client(
 
         raw_base_url = str(creds.get("base_url", "")).strip().rstrip("/") or pconfig.inference_base_url
         base_url = _to_openai_base_url(raw_base_url)
+        # Honour an explicit base_url override from the caller — used when a
+        # fallback_model entry (or custom_providers lookup) routes through a
+        # built-in provider name but targets a user-specified endpoint.
+        if explicit_base_url:
+            base_url = _to_openai_base_url(explicit_base_url.strip().rstrip("/"))
 
         default_model = _API_KEY_PROVIDER_AUX_MODELS.get(provider, "")
         final_model = _normalize_resolved_model(model or default_model, provider)