feat(proxy-rewrite): support NGINX variables in regex_uri

[ruanweihong-aaa]

Description

Currently, proxy-rewrite.uri supports NGINX variables, but the replacement part of proxy-rewrite.regex_uri only supports regular expression captures such as $1 and $2.

This PR adds NGINX variable resolution support to regex_uri replacements.

For example:

{
  "regex_uri": [
    "^/api/(.*)/users/(.*)$",
    "/v2/$1/external-users/$arg_name"
  ]
}

The replacement is processed in explicit stages:

1. Preserve literal dollar expressions such as $$x.
2. Resolve regular expression captures such as $1 and $2.
3. Resolve NGINX variables such as $arg_name and variables stored in ctx.var.
4. Restore literal dollar expressions.

Schema validation also distinguishes NGINX variables from regular expression captures. This allows a route containing both forms to be saved successfully.

Existing $1 and $2 capture behavior and $$ literal-dollar behavior remain compatible.

The English and Chinese proxy-rewrite documentation have also been updated to describe the new behavior.

Which issue(s) this PR fixes:

Fixes #13140

Tests

• Added t/plugin/proxy-rewrite4.t.
• Verified regular expression captures and NGINX variables in the same replacement.
• Verified existing $1 and $2 capture behavior.
• Verified literal $$x behavior.
• t/plugin/proxy-rewrite4.t: 6/6 passed.
• luacheck: 0 warnings and 0 errors for the modified Lua file.
• lj-releng: passed for the modified Lua file.
• Source-matched Docker BEFORE/AFTER verification: all six checks passed.

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

[nic-6443]

Why is this needed?

[ruanweihong-aaa]

This is needed because check_schema compiles the replacement with ngx.re.sub to validate both the regex pattern and replacement syntax.

Without escaping NGINX variables, $arg_name is interpreted as a named PCRE capture and route creation fails with "failed to compile the replacement template", which is the original issue.

Escaping only NGINX variable markers as $$ allows PCRE to keep validating regular captures such as $1 and invalid replacement syntax, while treating $arg_name as a literal during schema validation. The existing invalid replacement test also depends on this validation.

I can add a comment here to explain this.

[ruanweihong-aaa]

Added an inline comment in d1b362b explaining that schema validation must keep validating PCRE captures without treating NGINX variables as named captures.

[nic-6443]

It's too complicated. Is there a simpler way?

[ruanweihong-aaa]

The additional steps preserve three different replacement semantics:

1. $1 and $2 are regex captures.
2. $arg_name and values from ctx.var are NGINX variables.
3. $$x must remain the literal string $x.

Resolving NGINX variables directly would also consume $1, while letting ngx.re.sub process the replacement after variable resolution could interpret dollar signs from resolved values again.

I also tested resolving variables after ngx.re.sub, but at that point an escaped NGINX variable and an existing literal $$x both become $..., so they can no longer be distinguished.

I can move these stages into a focused helper to make the rewrite path easier to read. Would that address the concern, or would you prefer a different replacement behavior for literal $$?