feature: expose last upstream send state to balancer FFI

[membphis]

Summary

• add a balancer peer snapshot for the previous upstream send state
• expose ngx_http_lua_ffi_balancer_get_last_send_state() for lua-resty-core
• add focused FFI coverage for first-attempt, zero-byte retry, and completed-body retry states

Notes

• Refs 新增 ngx.balancer API 暴露上游重试发送状态 #3
• Local Test::Nginx execution was skipped as requested; CI is the source of truth for runtime verification.
• The user-facing ngx.balancer.get_last_send_state() wrapper will be implemented in the paired membphis/lua-resty-core PR.

Verification

• git diff --check
• PERL5LIB=/tmp/codex-perl5/lib/perl5 perl -I/tmp/codex-test-nginx/lib -c t/194-balancer-last-send-state.t