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/codex.rs

@@ -4400,7 +4400,7 @@ async fn built_tools(
         None
     };
 
-    if turn_context.config.features.enabled(Feature::SearchTool) {
+    if turn_context.config.features.enabled(Feature::Apps) {
         let mut selected_mcp_tools =
             if let Some(selected_tools) = sess.get_mcp_tool_selection().await {
                 filter_mcp_tools_by_name(mcp_tools.clone(), &selected_tools)