[codex] recover OpenClaw prompt metadata sessions

[NitayRabi]

Summary

• recover from OpenClaw ACP metadata failures that occur during the prompt process, not only during sessions ensure
• retain streaming stderr in returned errors so provider init failures can be detected without leaking stderr into stdout/chat events
• add a regression test using the production OpenClaw error shape, including the next: guidance text
• update the past-failures note to describe both recovery paths

Root cause

The existing recovery only inspected errors from acpx <agent> sessions ensure. OpenClaw can also fail later when the JSON prompt command initializes the ACP session, and streaming runs previously discarded stderr entirely. That meant the runner returned only run acpx: exit status 1, so the metadata recovery matcher never saw ACP_SESSION_INIT_FAILED.

Validation

• go test ./internal/acpx ./internal/app
• go test ./...