feat: populate upstream nginx variables when ai-proxy uses cosocket transport

[nic-6443]

Description

When the ai-proxy plugin sends requests via cosocket (resty.http), nginx's upstream module is bypassed entirely, leaving $upstream_status, $upstream_addr, $upstream_response_time, $upstream_header_time, $upstream_connect_time, $upstream_response_length, $upstream_host, $upstream_scheme, and $upstream_uri empty or defaulted in access logs.

This uses the FFI functions from apisix-nginx-module 1.19.4 (push_upstream_state / update_upstream_state) to populate these variables from Lua after cosocket requests complete.

Changes

Transport layer (ai-transport/http.lua)

• Capture timing info (connect_time, header_time, total time) during the HTTP lifecycle
• Attach upstream metadata (addr, host, scheme, uri, timings) to the response object
• Close cosocket on all error paths (connect failure, encode failure, request failure)

Plugin layer (ai-proxy/base.lua)

• Call push_upstream_state() after getting a response (or on transport errors) to populate addr, status, connect_time, header_time
• Call update_upstream_state() after body is fully consumed to populate response_time and response_length
• Set upstream_uri, upstream_host, and upstream_scheme to reflect the actual LLM provider endpoint
• Close cosocket on early return paths (429/5xx, missing body_reader)
• Works for both ai-proxy and ai-proxy-multi

Runtime

• Bump apisix-runtime to 1.3.5 (includes apisix-nginx-module 1.19.4)

Docs

• Document the newly populated upstream variables in the ai-proxy access log section
• Update the example access log entry to show populated values

Tests

• t/plugin/ai-proxy-upstream-vars.t: New test validating upstream variable population (addr, status, response_time, uri, host) for streaming and non-streaming
• Updated t/plugin/ai-proxy3.t: Adjusted access_log regex patterns to match populated upstream variables

[Copilot]

Pull request overview

This PR ensures standard Nginx upstream variables are populated for ai-proxy requests sent via cosocket (resty.http), so access logs (and any log consumers) can report upstream address/status/timings even though Nginx’s upstream module is bypassed.

Changes:

• Capture upstream metadata + timings in ai-transport/http.lua and propagate them via the response object (and via an error metadata return value on failures).
• Populate Nginx upstream state/vars from ai-proxy/base.lua using resty.apisix.upstream (push on response/error; update after body consumption).
• Update docs and tests to reflect/log newly populated upstream variables; bump APISIX runtime.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
apisix/plugins/ai-transport/http.lua	Captures connect/header timing and attaches upstream metadata to the cosocket response (and returns metadata on failure).
apisix/plugins/ai-proxy/base.lua	Pushes/updates upstream state for access logging and sets $upstream_scheme/$upstream_host/$upstream_uri based on the provider endpoint.
t/plugin/ai-proxy3.t	Updates access_log regex expectations to include upstream addr/status and the upstream URL variables.
t/plugin/ai-proxy-upstream-vars.t	Adds a new test intended to validate upstream variable population for streaming and non-streaming.
docs/en/latest/plugins/ai-proxy.md	Documents upstream variables being populated and updates the access-log example.
.requirements	Bumps APISIX_RUNTIME to 1.3.5.

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[Baoyuantop]

The PR description declares upstream_response_length, but it doesn't appear to be implemented in the code?