Resolve review feedback on PR #5313:
- Read reasoning_efforts from named custom_providers entries for custom:<name>
providers instead of looking only in providers:<name>.
- Preserve the nested-route deny ordering before the provider-config shortcut
so Gemini image/embedding routes cannot be re-enabled by config.
- Deduplicate configured effort values before returning them.
Add focused regression tests for bare provider config, named custom provider
config, all-invalid fallthrough, ACP hard guards, and nested route denies.
Charles Inglis
4f4c8fd183