[codex] fix debug wait server readiness status

[thatssomoneybaby]

Summary

• require successful HTTP responses for cortex debug wait --server-ready
• add a regression test covering a 404 readiness endpoint

Root Cause

The wait handler treated all client-error responses as readiness success, so a missing readiness endpoint could make --server-ready exit successfully.

Validation

• git diff --check
• cargo fmt --check --package cortex-cli
• cargo check -p cortex-cli
• cargo test -p cortex-cli --test debug_wait -- --nocapture

Related bounty issue: PlatformNetwork/bounty-challenge#46738