refactor(ai-aws-content-moderation): moderate decoded LLM content in access phase

[shreemaan-abhishek]

Description

Refactor ai-aws-content-moderation to be structured like its sibling ai-aliyun-content-moderation.

Previously the plugin ran in the rewrite phase (before ai-proxy) and sent the raw HTTP request body to AWS Comprehend. As a result Comprehend scored the undecoded JSON envelope (e.g. the literal escaped string "content":"toxic" and {"model":...,"messages":[...]}), while the upstream LLM acts on the decoded prompt, so the two saw different text.

What changed

• Move the plugin from the rewrite phase to the access phase (priority 1050 -> 1031, below ai-proxy's 1040), so it runs after ai-proxy and can reuse ctx.ai_client_protocol and ctx.picked_ai_instance.
• Make it protocol-aware: parse the JSON body and extract the LLM-visible prompt content via the detected protocol (ai-protocols), sending only the normalized, decoded content to Comprehend. This is what the sibling ai-aliyun-content-moderation plugin does.
• Guard on ctx.picked_ai_instance via fail_mode, so the plugin reports clearly when it is used without ai-proxy/ai-proxy-multi.
• Return a provider-compatible deny response (via ai-protocols) instead of a raw text body, so AI clients are not broken. Adds check_request, deny_code (default 200) and deny_message options.

The AWS Comprehend decision model (moderation_categories per-category thresholds and the overall moderation_threshold) is unchanged.

Behavior change

The plugin now must be used together with ai-proxy/ai-proxy-multi, consistent with ai-aliyun-content-moderation and with how the plugin is documented. A request that does not pass through ai-proxy is governed by fail_mode (default skip, i.e. passes through unchecked; set fail_mode: error to reject). This supersedes #13528.

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)