fix: preserve credential_pool through smart routing and defer eager fallback on 429 (#4361)

Three bugs prevented credential pool rotation from working when multiple
Codex OAuth tokens were configured:

1. credential_pool was dropped during smart model turn routing.
   resolve_turn_route() constructed runtime dicts without it, so the
   AIAgent was created without pool access. Fixed in smart_model_routing.py
   (no-route and fallback paths), cli.py, and gateway/run.py.

2. Eager fallback fired before pool rotation on 429. The rate-limit
   handler at line ~7180 switched to a fallback provider immediately,
   before _recover_with_credential_pool got a chance to rotate to the
   next credential. Now deferred when the pool still has credentials.

3. (Non-issue) Retry budget was reported as too small, but successful
   pool rotations already skip retry_count increment — no change needed.

Reported by community member Schinsly who identified all three root
causes and verified the fix locally with multiple Codex accounts.

gateway/run.py

@@ -788,6 +788,7 @@ class GatewayRunner:
             "api_mode": runtime_kwargs.get("api_mode"),
             "command": runtime_kwargs.get("command"),
             "args": list(runtime_kwargs.get("args") or []),
+            "credential_pool": runtime_kwargs.get("credential_pool"),
         }
         return resolve_turn_route(user_message, getattr(self, "_smart_model_routing", {}), primary)