Two callers of get_custom_provider_pool_key compared against its single
preferred key and broke when the pool held the other identity:
- _prune_replaced_custom_model_config_credentials skipped only the
preferred key, so a keyed provider's own legacy-named pool
(custom:b.ai) was false-pruned of its current model_config credential
when the preferred key resolved to the bare slug (b-ai).
- _seed_custom_pool seeded only when the pool key equaled the preferred
key, so a legacy-named pool stopped being seeded from model.api_key.
Both now compare against the full custom_provider_pool_key_candidates
set. Also drops a redundant get_custom_provider_pool_key call from
_try_resolve_from_custom_pool (it returned candidates[0], doubling the
config traversal) and updates the two test files that monkeypatched the
removed module attribute.
Follow-up to #100413.
kshitijk4poor