Speed up subagent startup

[daveaitel-openai]

Summary

• skip online model refresh for subagent sessions
• avoid rollout flushes during subagent startup
• keep /models refresh for non-subagent sessions

Testing

• cargo test -p codex-core --test all suite::models_etag_responses::refresh_models_on_models_etag_mismatch_and_avoid_duplicate_models_fetch
• cargo test -p codex-core --test all suite::remote_models::remote_models_long_model_slug_is_sent_with_high_reasoning
• cargo test -p codex-core --test all suite::model_switching::model_switch_to_smaller_model_updates_token_context_window
• cargo test -p codex-core --test all suite::compact::pre_sampling_compact_runs_on_switch_to_smaller_context_model
• cargo test -p codex-core --test all suite::compact::pre_sampling_compact_runs_after_resume_and_switch_to_smaller_model
• cargo test -p codex-core --test all suite::personality::remote_model_friendly_personality_instructions_with_feature

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

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