fix: resolve delegation providers from custom_providers

[stablegenius49]

Summary

• resolve named custom providers from config.yaml custom_providers during runtime provider selection
• allow delegation config to reference either the raw custom provider name (for example "local") or the menu-key form ("custom:local")
• add regression coverage for saved custom-provider credentials and env-key fallback

Testing

• python3 -m pytest -q -o addopts='' tests/test_runtime_provider_resolution.py
• python3 -m pytest -q -o addopts='' tests/test_api_key_providers.py::TestRuntimeProviderResolution

Closes #1011.