feat: add max_post_args_readable_size to bound post_arg.* body reads

[shreemaan-abhishek]

Description

Route matching supports post_arg.* predicates. For application/json and multipart/form-data requests, resolving these predicates in apisix/core/ctx.lua calls request.get_request_body_table() with no size limit, so the full request body (including temp-file-backed bodies) is parsed into worker memory during the access-phase matching.

This PR adds a configurable bound:

• New config apisix.max_post_args_readable_size (in MB, default 64, 0 disables the limit).
• post_arg.* matching now passes this cap to the body reader. Bodies larger than the cap are not read; the predicate resolves to nil and the route simply does not match.
• The cap covers the JSON and multipart paths (x-www-form-urlencoded already goes through nginx's bounded get_post_args).

Added schema validation, updated config.yaml.example, the router-radixtree docs, and tests in t/core/ctx3.t (oversized body is not read and the route does not match; within-cap body still matches).

Which issue(s) this PR fixes:

Fixes #

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)