bug: ai-proxy-multi plugin fails with plugin_config_id when multiple instances are configured

[chryancclee-boop]

Current Behavior

When using plugin_config_id to reference an external PluginConfig containing ai-proxy-multi with multiple instances, APISIX returns 503 with error "failed to fetch the parent config". Single instance works fine, and embedding multiple instances directly in the Route also works.

Expected Behavior

Multi-instance PluginConfig should work via plugin_config_id reference

Error Logs

*138335 [lua] resource.lua:63: fetch_latest_conf(): unsupported resource type: plugin_configs, client: 172.22.0.1, server: _, request: "POST /v1/chat/completions/test-multi HTTP/1.1", host: "127.0.0.1:9080", request_id: "3b90a7b3b15772d42b8b57fed5b6350e"

Steps to Reproduce

1. Create PluginConfig with ai-proxy-multi containing 2+ instances
2. Create Route referencing it via plugin_config_id
3. Send request → 503 "failed to fetch the parent config"

Environment

• APISIX version (run apisix version): 3.16.0
• Operating system (run uname -a): Linux 198ca90b5de7 6.8.0-110-generic test: missing no_shuffle() in node/sanity.t test case and added more step information. #110-Ubuntu SMP PREEMPT_DYNAMIC Thu Mar 19 15:09:20 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version (run openresty -V or nginx -V):
• etcd version, if relevant (run curl http://127.0.0.1:9090/v1/server_info):
• APISIX Dashboard version, if relevant:
• Plugin runner version, for issues related to plugin runners:
• LuaRocks version, for installation issues (run luarocks --version):