feat(plugin): error-page: support 403 and Nginx variables in body

[baiuu]

Description

Fixes #13742

Description

Two enhancements to the error-page plugin:

1. Support custom error pages for 403 responses

• metadata_schema: add error_403 (same structure as 404/500/502/503)
• get_metadata: lower status threshold from < 404 to < 400
• Behavior for unconfigured status codes is unchanged (lookup is still per error_<status> key)

2. Support Nginx variables in the error page body

• Resolve variables in header_filter via core.utils.resolve_var (same semantics as response-rewrite headers: $var, ${var}, ?? default values, \$ escaping)
• The rendered body is cached in ctx and emitted by body_filter
• Content-Length is set from the rendered body to avoid truncation or stalls
• Fail-open: on resolution error the original body is used and a warning is logged
• Bodies without $ short-circuit before regex processing — no overhead for existing configurations

Motivation: allow operators to serve unified custom error pages for auth rejections (403) and embed the request ID ($apisix_request_id) so users can report it and admins can correlate the page with logs.

Checklist

• New feature (non-breaking change which adds functionality)
• Added test cases in t/plugin/error-page.t:
  • TEST 21–24: custom 403 page via metadata; plugin-injected 403 (fault-injection) is intercepted
  • TEST 25–30: variable resolution ($var, ${var}), ?? default value, \$ escaping
• Updated documentation (EN + ZH)
• CI passed

Acknowledgement

This change was developed with the assistance of Kimi K3 (Moonshot AI) — including implementation, test cases, and documentation.