The Bedrock API-key flow stored the bearer token in OPENAI_API_KEY and set
a bare `provider: custom`. Since #28660 that variable is only honoured for
openai.com hosts, so for bedrock-mantle.*.api.aws the token was dropped and
requests went out with api_key="no-key-required", a 401 on every call.
Write a named `providers.bedrock-mantle` entry with
key_env: AWS_BEARER_TOKEN_BEDROCK instead. The named-provider branch in
runtime_provider.py resolves key_env; the bare-custom branch cannot.
Fixes authentication only. Per-model mantle route selection is separate.
Darko Mesaroš