fix(gateway): isinstance-guard string-form 429 error body

When a non-Anthropic provider (e.g. Morpheus proxy) returns a 429 with
`{"error": "Too Many Requests"}` instead of the expected
`{"error": {"type": ...}}` dict, _err_body.json().get("error", {})
returns the raw string and the next .get("type") line crashes with
AttributeError, taking down the message handler.

Guard with isinstance(_err_json, dict) so non-dict error bodies fall
through to the generic rate-limit hint.

Salvaged from PR #2587 by @KiraKatana. The PR's fallback-config
`base_url`/`api_key_env` fix was already implemented independently
on main (run_agent.py:8759-8780) with additional aliases and Ollama
Cloud host handling, so only the gateway guard is cherry-picked.

Co-authored-by: KiraKatana <kira.ops@proton.me>

gateway/run.py

@@ -7991,6 +7991,8 @@ class GatewayRunner:
                 try:
                     if _err_body is not None:
                         _err_json = _err_body.json().get("error", {})
+                        if not isinstance(_err_json, dict):
+                            _err_json = {}
                 except Exception:
                     pass
                 if _err_json.get("type") == "usage_limit_reached":