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/app-server/src/request_processors/config_processor.rs

@@ -53,7 +53,6 @@ const SUPPORTED_EXPERIMENTAL_FEATURE_ENABLEMENT: &[&str] = &[
     "mentions_v2",
     "plugins",
     "remote_control",
-    "tool_search",
     "tool_suggest",
     "tool_call_mcp_elicitation",
 ];