[I] feat(plugin): error-page: support 403 and Nginx variables in body [apisix]

["baiuu (via GitHub)" <gi...@apache.org>]

baiuu opened a new issue, #13742:
URL: https://github.com/apache/apisix/issues/13742

   ### Description
   
   ### Description
   
   The `error-page` plugin currently has two limitations that make custom error pages less useful in production:
   
   **1. Limited status code coverage.** Only 404/500/502/503 are supported. APISIX-generated 403 responses are common — auth plugin rejections (key-auth, jwt-auth, etc.), ACL blocks, or custom serverless logic — but they cannot be replaced with a custom page.
   
   **2. Static body only.** The configured `body` is returned as a static string. Operators typically want to embed request-scoped information into error pages — most importantly the **request ID**, so that end users can report it and administrators can correlate the error page with access/error logs.
   
   ### Proposal
   
   **Part 1 — Support 403:**
   - Add `error_403` to `metadata_schema` (same structure as existing entries)
   - Lower the status threshold in `get_metadata` from `status < 404` to `status < 400`; interception still requires an explicitly configured `error_<status>` key, so behavior for unconfigured codes is unchanged
   
   **Part 2 — Nginx variables in body:**
   - Resolve variables via `core.utils.resolve_var` (the same mechanism used by `response-rewrite` headers) before sending the response
   - Resolution happens in `header_filter` so `Content-Length` is computed from the **final rendered body** (avoids truncation/connection stalls)
   - Supports `$var`, `${var}`, the `??` default-value operator, and `\$` escaping
   - Fail-open: on resolution failure, fall back to the original body and log a warning
   - Zero overhead for existing configurations (bodies without `$` short-circuit before regex processing)
   
   Example:
   
   ```json
   {
       "enable": true,
       "error_403": {
           "body": "<html><body><h1>403 Forbidden</h1><p>Request ID: $apisix_request_id</p></body></html>",
           "content_type": "text/html"
       }
   }
   ```
   
   Both parts are backward compatible and independent of each other, but they combine naturally — e.g. a custom 403 page showing `$apisix_request_id` for troubleshooting auth rejections.
   


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