feat: support backlog in listen and if= condition in access_log via nginx_config

[Ethan-Zhang-ZH]

Description

Please refer to #14022. This PR implements both enhancements proposed there.

1. backlog for listen sockets

nginx_config:
  http:
    node_listen:
      - ip: 0.0.0.0
        port: 9080
        backlog: 8192   # optional, absent -> rendered as today (nginx default 511)

Renders the backlog=N parameter of the nginx listen directive, so operators can
size the pending connections queue from config.yaml instead of relying on the default
511 (which can overflow under connection spikes even when host-level
net.core.somaxconn is tuned).

2. if= condition for access_log

nginx_config:
  http:
    access_log_if: $loggable   # optional, absent -> rendered as today
    http_configuration_snippet: |
      map $status $loggable {
          default 1;
      }

Renders an if=<condition> parameter on the access_log directive. The condition is
evaluated in the log phase and can reference variables declared via
http_configuration_snippet (map / split_clients), enabling log sampling or
per-status / per-endpoint log exclusion with plain nginx config, without any Lua plugin.
It is also the generic fix for #10407.

Changes

• apisix/cli/ops.lua: pass an optional, validated backlog (positive integer) through listen_table_insert
• apisix/cli/ngx_tpl.lua: render backlog=N and if=<condition> when configured (output is byte-identical when absent)
• conf/config.yaml.example: document both fields
• t/cli/test_main.sh: backlog rendering + invalid-value test cases
• t/cli/test_access_log.sh: if= rendered / not-rendered test cases

Both fields are optional and fully backward compatible. All t/cli suites touched by
these changes pass locally.

Fixes #14022