[codex] Use gpt-5.4-mini by default

[timrogers]

Summary

• change the default OpenAI model from gpt-4o to gpt-5.4-mini

Why

• align the built-in OpenAI default with the requested newer model

Impact

• OpenAI-backed inference now uses gpt-5.4-mini unless overridden elsewhere

Validation

• cargo build --release --locked
• cargo test --locked --all-features (fails in this environment because mcp::tests::login_complete_stores_token_and_updates_server_state cannot bind a local httpmock server: Operation not permitted)