docs(ai-proxy): document client request header forwarding behavior

[shreemaan-abhishek]

Description

The ai-proxy and ai-proxy-multi plugins forward the incoming client request headers to the configured LLM upstream by default, dropping only Host, Content-Length, and Accept-Encoding and forcing Content-Type to application/json. This behavior is not currently described in the plugin documentation.

This PR documents the behavior in a new "Request Header Forwarding" section for both plugins (English and Chinese docs). It explains:

• which headers are dropped and which are forced;
• that headers configured under auth.header take precedence over client headers of the same name;
• that, since the LLM upstream is often a third-party service, operators should strip headers they do not want to expose (for example Authorization or Cookie) before the request reaches the plugin, e.g. with proxy-rewrite.

Documentation only. No code or behavior changes.

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)