[codex] Keep extension tools directly visible

[jif-oai]

Summary

Keep extension-owned tools directly model-visible even when their shared executor reports ToolExposure::Deferred.

Root cause

ExtensionToolAdapter started forwarding extension executor exposure, but extension adapters do not yet provide search_info(). A deferred extension tool could therefore disappear from the model-visible tool list without becoming discoverable through tool_search.

Changes

• Force ExtensionToolAdapter exposure back to Direct until extension-backed search metadata exists.
• Add a spec-planning regression test proving a deferred extension executor still appears in the model-visible tool specs.

Validation

• just fmt
• cargo test -p codex-core deferred_extension_tools_remain_model_visible
• cargo test -p codex-core hit an existing unrelated stack overflow in agent::control::tests::resume_agent_from_rollout_skips_descendants_when_parent_resume_fails.
• just fix -p codex-core