[codex] Fix agent identity auth test fixture

[adrian-openai]

Summary

• Add the missing background_task_id: None field to the AgentIdentityAuthRecord fixture introduced in auth_tests.rs.

Why

• Current main fails Bazel/rust-ci compile paths after the background-task auth field landed and a later auth test fixture constructed AgentIdentityAuthRecord without that new field.
• I intentionally removed the earlier broader CI-stability edits from this PR. The code-mode timeout, external-agent migration snapshot, and MCP resource timeout failures appear to be general/flaky or unrelated to the agent identity merge stack rather than cleanly caused by it.

Validation

• cargo test -p codex-login dummy_chatgpt_auth_does_not_create_cwd_auth_json_when_identity_is_set -- --nocapture
• just fmt