Improve missing API key error: avoid ENOENT on missing server info JSON

Summary

When openai/codex-action@v1 runs without an openai-api-key value, it fails with a low-signal ENOENT error about a missing server info JSON file.

Actual behavior

The action fails at read-server-info with:

Error reading server info: Error: ENOENT: no such file or directory, open '/home/runner/.codex/<run_id>.json'
Error: Failed to read server info from /home/runner/.codex/<run_id>.json

In this case, the root problem was that secrets.OPENAI_API_KEY was not set in the repo.

Expected behavior

A clear failure message like:

• openai-api-key input is empty (did you forget to set OPENAI_API_KEY secret?)

instead of a downstream file-not-found error.

Why this is confusing

ENOENT on an internal .codex/<run_id>.json file reads like a runtime crash, not a missing input. It makes debugging much harder than necessary.

Suggested fix

Either:

1. Add an explicit preflight validation for missing openai-api-key when required, or
2. Guard read-server-info so it cannot run unless proxy startup/server-info creation was attempted.

Start by tracing the action's handling of the openai-api-key input and the read-server-info step, then inspect how proxy startup and server-info creation are sequenced. Reproduce the action without the input and verify that it reports the missing key clearly instead of the downstream ENOENT error.