fix: enhance litellm parameter dropping for codex compatibility

Add router_settings and default_litellm_params to ensure unsupported
parameters like prompt_cache_key are properly dropped when using codex
with the litellm proxy.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

ai/litellm-config.yaml

@@ -28,5 +28,12 @@ litellm_settings:
   drop_params: true
   set_verbose: true
 
+router_settings:
+  allowed_fails: 0
+
+# Drop unsupported parameters
+default_litellm_params:
+  drop_params: true
+
 general_settings:
   disable_responses_id_security: true