test: cover nonstream upstream failed response shape

[HisenWeb]

Summary

Adds P1-3 regression coverage for non-stream upstream HTTP errors.

After inspection, the runtime already returns a Responses-shaped body with status: failed for non-stream /v1/responses upstream failures. This PR therefore locks the behavior with a focused regression test instead of changing runtime code.

Changes

• Adds tests/nonstream_upstream_error_regression.rs.
• Verifies upstream HTTP errors produce a Responses object with:
  • object: response
  • status: failed
  • empty output
  • zeroed usage
  • original model alias and metadata
  • error.type/code: upstream_error
• Adds docs/p1-nonstream-upstream-error.md documenting the current behavior and the future HTTP-200 policy option if real Codex validation requires it.

Current policy

The adapter currently preserves the upstream HTTP status while returning a Responses failed body. If real Codex subagent validation shows that non-2xx status still breaks the chain, the next policy change should be explicit: return HTTP 200 with response.status = failed for /v1/responses upstream errors.

Validation

Edited via GitHub connector. Please run locally before merge:

cargo fmt --check
cargo test --test nonstream_upstream_error_regression
cargo test --test test_e2e
cargo test

Notes

This PR intentionally does not change /v1/models upstream error behavior.

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