feat: persist and restore codex app's tools after search (#11780)

### 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_id`s
   - 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.

codex-rs/core/tests/common/lib.rs

@@ -12,6 +12,7 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 use regex_lite::Regex;
 use std::path::PathBuf;
 
+pub mod apps_test_server;
 pub mod context_snapshot;
 pub mod process;
 pub mod responses;