fix(delegation): preserve configured_provider name when runtime returns 'custom'

Named custom providers (e.g. crof.ai) resolve to provider='custom' at the
runtime level, causing subagents to lose their intended provider identity.
On retry/fallback, resolve_provider_client('custom', model=...) searches all
providers advertising that model and picks non-deterministically, routing to
Z.AI or Bailian instead of the configured target.

The fix preserves configured_provider when runtime['provider'] == 'custom',
restoring the original provider name so routing stays correct through retries.
Adds a named constant _RUNTIME_PROVIDER_CUSTOM instead of a magic string.

Adds three regression tests:
- test_named_custom_provider_preserves_provider_name: the #26954 case
- test_standard_provider_not_overwritten_by_configured_name: openrouter/nous
  must still return their own identity, not the configured name
- test_custom_provider_with_empty_configured_provider_falls_back_to_runtime:
  empty provider triggers the early-return None path as before

tools/delegate_tool.py

@@ -31,6 +31,11 @@ from concurrent.futures import (
 from typing import Any, Dict, List, Optional
 
 from toolsets import TOOLSETS
+
+# Sentinel value used by the runtime provider system for providers that are
+# not natively known (named custom providers, third-party aggregators, etc.).
+# Must match hermes_cli.runtime_provider.RUNTIME_PROVIDER_TYPE_CUSTOM.
+_RUNTIME_PROVIDER_CUSTOM = "custom"
 from tools import file_state
 from tools.terminal_tool import set_approval_callback as _set_subagent_approval_cb
 from utils import base_url_hostname, is_truthy_value
@@ -2442,7 +2447,7 @@ def _resolve_delegation_credentials(cfg: dict, parent_agent) -> dict:
 
     return {
         "model": configured_model or runtime.get("model") or None,
-        "provider": runtime.get("provider"),
+        "provider": configured_provider if runtime.get("provider") == _RUNTIME_PROVIDER_CUSTOM else runtime.get("provider"),
         "base_url": runtime.get("base_url"),
         "api_key": api_key,
         "api_mode": runtime.get("api_mode"),