[PR] fix(ai-aws-content-moderation): moderate decoded LLM content instead of raw body [apisix]

["shreemaan-abhishek (via GitHub)" <gi...@apache.org>]

shreemaan-abhishek opened a new pull request, #13528:
URL: https://github.com/apache/apisix/pull/13528

   ### Description
   
   The `ai-aws-content-moderation` plugin sent the **raw** HTTP request body to AWS Comprehend instead of parsing the LLM request and extracting the actual prompt content. As a result, moderation scored the undecoded JSON envelope, while the upstream LLM acts on the decoded content, so the two see different text:
   
   - A body containing `"content":"toxic"` was scored by Comprehend as the literal escaped string, whereas the upstream model decodes it to `toxic`.
   - The full `{"model":...,"messages":[...]}` envelope was scored as-is, adding noise to the toxicity result.
   
   This is inconsistent with the sibling `ai-aliyun-content-moderation` plugin, which is protocol-aware.
   
   #### What changed
   
   Make the plugin protocol-aware, in the same style as `ai-aliyun-content-moderation`:
   
   - Require/validate the `application/json` content type.
   - Parse the JSON body, detect the client protocol via `ai-protocols`, and extract the LLM-visible content (`messages[].content`, Responses input/instructions, Anthropic message content, etc.).
   - Send only the normalized, decoded content to Comprehend.
   
   Because the plugin runs in the `rewrite` phase (before `ai-proxy`), it detects the protocol directly rather than relying on `ctx.ai_client_protocol`. Requests that are not recognized AI requests (non-JSON content type, unparseable bodies, or JSON that carries no LLM content) are handled by the existing `fail_mode` (`skip` by default), so non-AI traffic on Consumer-bound plugins keeps its current pass-through behavior.
   
   #### Behavior change
   
   Making the plugin JSON/LLM-only is a behavior change for anyone running it on non-JSON routes. With the default `fail_mode: skip`, such requests now pass through unchecked instead of being forwarded verbatim to Comprehend; set `fail_mode: error` to reject them.
   
   #### Which issue(s) this PR fixes:
   
   Fixes #
   
   ### Checklist
   
   - [x] I have explained the need for this PR and the problem it solves
   - [x] I have explained the changes or the new features added to this PR
   - [x] I have added tests corresponding to this change
   - [x] I have updated the documentation to reflect this change
   - [x] I have verified that this change is backward compatible (If not, please discuss on the [APISIX mailing list](https://github.com/apache/apisix/tree/master#community) first)


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