Remove ToolSearch feature toggle (#23389)

## Summary
- mark `ToolSearch` as removed and ignore stale config writes for its
legacy key
- make search tool exposure depend only on model capability, not a
feature toggle
- remove app-server enablement support and prune now-obsolete test
coverage/setup

## Verification
- `cargo test -p codex-features`
- `cargo test -p codex-tools`
- `cargo test -p codex-core search_tool_requires_model_capability`
- `cargo test -p codex-app-server experimental_feature_enablement_set_`

## Notes
- This keeps the legacy config key as a no-op for compatibility while
removing the ability to toggle the behavior off cleanly.
- No developer-facing docs update outside the touched app-server README
was needed.

codex-rs/tools/src/tool_config.rs

@@ -182,8 +182,7 @@ impl ToolsConfig {
         let include_multi_agent_v2 = features.enabled(Feature::MultiAgentV2);
         let include_collab_tools = include_multi_agent_v2 || features.enabled(Feature::Collab);
         let include_agent_jobs = features.enabled(Feature::SpawnCsv);
-        let include_search_tool =
-            model_info.supports_search_tool && features.enabled(Feature::ToolSearch);
+        let include_search_tool = model_info.supports_search_tool;
         let include_tool_suggest = features.enabled(Feature::ToolSuggest)
             && features.enabled(Feature::Apps)
             && features.enabled(Feature::Plugins);