Consolidate search_tool feature into apps (#11509)

## Summary
- Remove `Feature::SearchTool` and the `search_tool` config key from the
feature registry/schema.
- Gate `search_tool_bm25` exposure via `Feature::Apps` in
`core/src/tools/spec.rs`.
- Update MCP selection logic in `core/src/codex.rs` to use
`Feature::Apps` for search-tool behavior.
- Update `core/tests/suite/search_tool.rs` to enable `Feature::Apps`.
- Regenerate `core/config.schema.json` via `just write-config-schema`.

## Testing
- `just fmt`
- `cargo test -p codex-core --test all suite::search_tool::`

## Tickets
- None

codex-rs/core/src/tools/spec.rs

@@ -58,7 +58,7 @@ impl ToolsConfig {
         let include_collab_tools = features.enabled(Feature::Collab);
         let include_collaboration_modes_tools = features.enabled(Feature::CollaborationModes);
         let request_rule_enabled = features.enabled(Feature::RequestRule);
-        let include_search_tool = features.enabled(Feature::SearchTool);
+        let include_search_tool = features.enabled(Feature::Apps);
 
         let shell_type = if !features.enabled(Feature::ShellTool) {
             ConfigShellToolType::Disabled