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/features/src/tests.rs

@@ -188,9 +188,10 @@ fn network_proxy_is_experimental_and_disabled_by_default() {
 }
 
 #[test]
-fn tool_search_is_stable_and_enabled_by_default() {
-    assert_eq!(Feature::ToolSearch.stage(), Stage::Stable);
-    assert_eq!(Feature::ToolSearch.default_enabled(), true);
+fn tool_search_is_removed_and_disabled_by_default() {
+    assert_eq!(Feature::ToolSearch.stage(), Stage::Removed);
+    assert_eq!(Feature::ToolSearch.default_enabled(), false);
+    assert_eq!(feature_for_key("tool_search"), Some(Feature::ToolSearch));
 }
 
 #[test]
@@ -603,7 +604,6 @@ fn materialize_resolved_enabled_writes_all_features_and_preserves_custom_config(
     features.enable(Feature::CodeMode);
     features.enable(Feature::MultiAgentV2);
     features.enable(Feature::NetworkProxy);
-    features.disable(Feature::ToolSearch);
 
     let mut features_toml = FeaturesToml {
         multi_agent_v2: Some(FeatureToml::Config(crate::MultiAgentV2ConfigToml {