fix(ai-proxy-multi): keep the client request body intact across fallback retries

[janiussyafiq]

Description

The retry loop in before_proxy re-reads the parsed request body from the per-request cache (ctx._request_body_table), but build_request rewrites that shared table in place with per-instance options. A fallback attempt therefore starts from the previous instance's rewritten body: $request_llm_model is clobbered with the first instance's model, and the first instance's options (e.g. temperature) leak into the retry request sent to the fallback instance.

This PR deep-copies the parsed body at the start of every attempt so each instance builds its request from the client's original body, and drops the redundant request_llm_model re-assignment in before_proxy (detect_request_type already captures it from the client body in the access phase, for both ai-proxy and ai-proxy-multi).

The copy is O(body) and cheaper than the JSON decode of the same body already performed in the access phase. Keeping the shared cache pristine also fixes log-phase post_arg.* variable reads and the semantic balancer's re-pick on retry, both of which previously saw the instance-rewritten body instead of the client's request.

Which issue(s) this PR fixes:

Fixes #13769

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)

[membphis]

LGTM