revert: drop configurable request JSON library

[nic-6443]

Description

This PR reverts the previous configurable request JSON library change and keeps request body JSON handling on cjson only.

The previous switch to new JSON libraries was too aggressive. After more testing, the new libraries did not meet the stability bar we need for request-body parsing, so this PR removes the request_body_json_lib option, removes the simdjson/qjson dependencies and setup, and routes AI transport body encoding back through the existing core.json path.

Checklist

• Tests updated
• Documentation updated

Tests

• prove -Itest-nginx/lib -I. -r t/core/config-default.t t/core/request.t

[Copilot]

Pull request overview

This PR improves robustness of JSON request-body decoding when apisix.request_body_json_lib is set to simdjson by retrying with cjson after simdjson decode failures, aiming to keep simdjson as the fast path while preserving previous cjson compatibility.

Changes:

• Add a simdjson→cjson fallback path in apisix.core.request_json.decode() with a warning log on simdjson decode failures.
• Update api7-lua-resty-simdjson dependency to 0.1.2-1.
• Add/adjust tests and update ai-proxy documentation (EN/ZH) to reflect the fallback behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
apisix/core/request_json.lua	Adds simdjson decode wrapper that logs and falls back to cjson decoding on failure.
apisix-master-0.rockspec	Bumps api7-lua-resty-simdjson version to include decoder cleanup after errors.
t/core/request.t	Adds a regression test ensuring fallback to cjson occurs and warning is logged; renumbers subsequent tests.
docs/en/latest/plugins/ai-proxy.md	Documents warning + retry-with-cjson behavior for simdjson mode.
docs/zh/latest/plugins/ai-proxy.md	Same documentation update in Chinese.

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