fix: normalize qjson errors in request json

[nic-6443]

This follows up on the qjson review in #13386. qjson can raise Lua errors from decode/materialize/encode paths, while the request JSON helper callers expect the same nil, err shape as core.json.

This patch wraps the qjson paths and normalizes those failures to nil, err, while keeping cjson and simdjson behavior unchanged. It also adds a core request test for malformed request body decode and qjson encode failures.

Checks run:

• git diff --check
• luacheck apisix/core/request_json.lua
• perl -I. -c t/core/request.t
• focused runtime smoke for qjson decode/encode error normalization

[Copilot]

Pull request overview

This PR normalizes qjson failures in apisix.core.request_json so callers consistently receive nil, err instead of uncaught Lua errors, aligning qjson behavior with expectations established by core.json-style helpers.

Changes:

• Wrap qjson.decode, qjson.materialize, and qjson.encode with pcall and convert thrown errors into nil, err returns.
• Add a t/core/request.t test to assert malformed JSON request bodies and qjson encode/decode failures return nil, err without emitting [error] logs.
• Renumber subsequent request tests to account for the new coverage.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
apisix/core/request_json.lua	Adds guarded qjson decode/materialize/encode helpers to normalize thrown errors into nil, err.
t/core/request.t	Adds regression test coverage for qjson decode/encode error normalization and malformed JSON request body handling.

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