Context

PR #25530 adds per-call model/provider override for delegate_task, but leaves direct endpoint base_url override as a separate follow-up.

Work

• Add a safe per-call base_url override path for direct/custom endpoints.
• Validate interaction with provider/model override and credential handling.
• Add tests for explicit base_url, default behavior, and invalid endpoint inputs.

Source

Follow-up from #25530.

Referenced by #25530.