fix(delegate): resolve custom-endpoint subagent pools by endpoint identity (#41730)

Subagents delegated to a custom endpoint were misrouted when the parent
ran on a different custom endpoint. Both runtimes collapse to
provider="custom", so _resolve_child_credential_pool() treated them as
interchangeable and handed the child the parent's pool. Leasing from it
then overwrote the child's delegated base_url with the parent's endpoint
via _swap_credential() — the child sent the delegated model name to the
wrong endpoint.

Custom runtimes now resolve by endpoint identity (the custom:<name> pool
key derived from base_url). The parent pool is reused only when both
parent and child resolve to the same custom endpoint; unregistered raw
endpoints return None so the child keeps its fixed delegated credential.
Non-custom provider paths are unchanged.

Fixes #7833.

agent/credential_pool.py

@@ -374,7 +374,7 @@ def _iter_custom_providers(config: Optional[dict] = None):
         yield _normalize_custom_pool_name(name), entry
 
 
-def get_custom_provider_pool_key(base_url: str, provider_name: Optional[str] = None) -> Optional[str]:
+def get_custom_provider_pool_key(base_url: Optional[str], provider_name: Optional[str] = None) -> Optional[str]:
     """Look up the custom_providers list in config.yaml and return 'custom:<name>' for a matching base_url.
 
     When provider_name is given, prefer matching by name first (solving the case where