fix(ai-aliyun-content-moderation): return nil instead of ngx.OK in lua_body_filter

[janiussyafiq]

Description

ai-aliyun-content-moderation's lua_body_filter returned ngx.OK on the streaming final_packet path. In apisix/plugin.lua's lua_response_filter, returning a non-nil code prints the body and ngx.exits the whole response filter chain, so any lower-priority response body filter is skipped.

Because ai-aliyun-content-moderation (priority 1029) runs before ai-lakera-guard (priority 1028), a route chaining both terminated the chain before ai-lakera-guard's output scan ran — even though final_packet only annotates risk_level and never blocks. The result was a silent moderation gap: a streamed response that ai-lakera-guard would block could leak through unscanned.

This returns nil (transform-and-continue) instead of ngx.OK on the final_packet path, so the chain keeps running lower-priority response body filters. The paths that genuinely terminate the response (realtime block) still return a code and are unchanged.

A combined-plugin streaming test (t/plugin/ai-lakera-guard-chain.t) is added: it asserts ai-lakera-guard's output scan still runs and blocks a flagged stream when chained after ai-aliyun-content-moderation, and that a clean stream still passes through the chain.

Which issue(s) this PR fixes:

Fixes #13620

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

[membphis]

Code review looks good to me. Please make sure CI is green before merging.