fix(batch-requests): missing the port number error when APISIX listens on a unix socket

[AlinsRan]

Description

When APISIX listens on a unix domain socket (e.g. added via nginx_config.http_server_configuration_snippet), calling the batch-requests endpoint over that socket fails with:

{"error_msg":"connect to apisix failed: missing the port number"}

The plugin loops the pipelined requests back to APISIX itself with httpc:connect("127.0.0.1", ngx.var.server_port), but nginx renders $server_port as an empty string for a request that arrived over a unix socket. The empty string is neither a number nor a unix: prefixed host, so lua-nginx-module's tcpsock:connect() rejects it with missing the port number; resty.http passes that through and the plugin turns it into the 500 above.

This PR makes the loopback connect fall back to a TCP port from apisix.node_listen when $server_port is empty. The value read from the local conf is not normalized by the CLI, so both config forms are handled: a plain port number, and an array of port numbers or {ip, port} tables. Since the loopback connect targets 127.0.0.1, an entry bound to a specific external address is only used as a last resort — the first entry reachable over loopback (no ip, 0.0.0.0, or 127.0.0.1) wins. If no TCP port can be resolved at all, the plugin returns a 503 explaining that batch-requests needs a TCP listener to loop back requests.

An alternative would be to connect back over the unix socket itself, but the listening socket path is not exposed through any nginx variable, so that is left as possible future work.

This revives #13530, which was closed by its author while still a draft and never reviewed. Two changes relative to that branch:

• the node_listen entry selection is now address-aware, as described above; the original always took the first entry and could pick a port bound to an unreachable address
• the regression test builds the request body with core.json.encode instead of an inlined long-bracket JSON string, which can trip the nginx config parser when the block crosses a config-buffer boundary

Added a regression test that injects an extra listen unix: listener into the test server and sends the batch-requests call over it.

Test status: the code passes luajit -bl syntax checking and luacheck. I have not run the test-nginx suite locally; relying on CI for that.

Which issue(s) this PR fixes:

Fixes #11781

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)