feat: add direct endpoint overrides for auxiliary and delegation

Add base_url/api_key overrides for auxiliary tasks and delegation so users can
route those flows straight to a custom OpenAI-compatible endpoint without
having to rely on provider=main or named custom providers.

Also clear gateway session env vars in test isolation so the full suite stays
deterministic when run from a messaging-backed agent session.

website/docs/user-guide/features/delegation.md

@@ -209,6 +209,14 @@ Delegation has a **depth limit of 2** — a parent (depth 0) can spawn children
 delegation:
   max_iterations: 50                        # Max turns per child (default: 50)
   default_toolsets: ["terminal", "file", "web"]  # Default toolsets
+  model: "google/gemini-3-flash-preview"             # Optional provider/model override
+  provider: "openrouter"                             # Optional built-in provider
+
+# Or use a direct custom endpoint instead of provider:
+delegation:
+  model: "qwen2.5-coder"
+  base_url: "http://localhost:1234/v1"
+  api_key: "local-key"
 ```
 
 :::tip