feat: persist and restore codex app's tools after search

[apanasenko-oai]

What changed

1. Removed per-turn MCP selection reset in core/src/tasks/mod.rs.
2. Added SessionState::set_mcp_tool_selection(Vec<String>) in core/src/state/session.rs for authoritative restore behavior (deduped, order-preserving, empty clears).
3. Added rollout parsing in core/src/codex.rs to recover active_selected_tools from prior search_tool_bm25 outputs:
   • tracks matching call_ids
   • parses function output text JSON
   • extracts active_selected_tools
   • latest valid payload wins
   • malformed/non-matching payloads are ignored
4. Applied restore logic to resumed and forked startup paths in core/src/codex.rs.
5. Updated instruction text to session/thread scope in core/templates/search_tool/tool_description.md.
6. Expanded tests in core/tests/suite/search_tool.rs, plus unit coverage in:
   • core/src/codex.rs
   • core/src/state/session.rs

Behavior after change

1. Search activates matched tools.
2. Additional searches union into active selection.
3. Selection survives new turns in the same thread.
4. Resume/fork restores selection from rollout history.
5. Separate threads do not inherit selection unless forked.