hermes auth add stores providers.<key> credentials under the config
slug, but runtime only tried custom:<display-name> and then sent the
no-key-required placeholder. Try the slug first, keep the legacy
namespace as fallback, and thread provider_key/key_env through named
custom resolution.
Salvage of #93214 (5 commits squashed onto current main; agent_runtime_helpers.py
diverged since the PR base and was 3-way reapplied). The credential-rotation
guard in recover_with_credential_pool and both restore_primary_runtime paths
only tolerated the custom-naming split when the agent carried the literal label
'custom', so a named custom provider (agent.provider='gemini-no-filter', pool
'custom:gemini-no-filter') tripped the mismatch guard and skipped rotation on
every 401/429. Now all three guard sites use the canonical
credential_pool_matches_provider boundary predicate + resolve_runtime_pool_key,
which recognizes configured named-custom aliases and validates endpoints.
Fixes#93188.
xxxigm