fix: resolve delegation providers from custom_providers

[teknium1]

Summary

• salvage PR fix: resolve delegation providers from custom_providers #1012 onto current main by cherry-picking the contributor's runtime-provider fix with authorship preserved
• resolve delegation providers from config.yaml custom_providers when delegation uses either the raw saved name (for example local) or the menu-key form (custom:local)
• keep raw custom names from shadowing built-in provider ids like nous
• keep the regression coverage aligned with current main while retaining tests for saved credentials, env fallback, and builtin-vs-custom precedence

Test plan

• python -m pytest tests/test_runtime_provider_resolution.py tests/test_api_key_providers.py tests/tools/test_delegate.py -n0 -q
• python -m pytest tests/ -n0 -q

Closes #1011
Salvages #1012