feat: custom base_url implies bypass_models_gateway

[dgageot]

When a model is configured with a custom base_url — either set directly on the model or inherited from a named entry in the providers: section — it is clearly meant to reach a specific endpoint that is not the default for that provider. Routing such a model through a configured models gateway serves no purpose and often breaks things, yet until now callers had to remember to also set bypass_models_gateway: true explicitly.

This change makes the implication automatic. A custom (non-default) base_url now sets bypass_models_gateway: true at config resolution time, so the model dials its endpoint directly. Built-in provider alias defaults are unaffected. The predicate that decides whether a URL is "custom" is consolidated into latest.HasCustomBaseURL so the logic lives in one place. The env-var preflight now correctly requires direct credentials for these models even when a gateway is configured, and also expands ${env.X} refs that come from an inherited provider-level base_url. first_available candidates that bypass the gateway — whether via the explicit flag or the new implied path — are credential-checked against the direct endpoint rather than being selected blindly.

No breaking change to existing configs: anyone who already sets bypass_models_gateway: true alongside base_url will see identical behavior.

[docker-agent]

Assessment: 🟢 APPROVE

All findings from the drafter were dismissed upon verification. The logic in HasCustomBaseURL, the routerFallbackLeaf helper, and the first_available credential-checking path are all correct. The behavior change in gather.go (stripping router-level fields before passing the fallback leaf to env-var collection) is intentional and mirrors the runtime's actual behavior, as explicitly documented by the code comments.

Summary: The PR cleanly implements the implied-bypass invariant: a custom base_url on a model or inherited from a provider definition now automatically implies bypass_models_gateway: true. No regressions found.