[codex] Remove async_trait from ToolExecutor (#27304)

## Why

We're now [discouraging use of
`async_trait`](https://github.com/openai/codex/pull/20242).

Removing use of `async_trait` from `ToolExecutor` yields a `codex_core`
debug test build speedup of ~78% (from 227.5s to 50.3s) on my machine.

Stacked on #27299, this PR applies the trait change after the handler
bodies have been outlined.

## What

Changed `ToolExecutor::handle` to return an explicit boxed
`ToolExecutorFuture` instead of using `async_trait`.

Updated ToolExecutor implementors to return `Box::pin(...)`, reexported
the future alias through `codex-tools` and `codex-extension-api`, and
removed `codex-tools` direct `async-trait` dependency.

codex-rs/tools/src/lib.rs

@@ -93,6 +93,7 @@ pub use tool_discovery::ToolSearchSourceInfo;
 pub use tool_discovery::collect_request_plugin_install_entries;
 pub use tool_discovery::filter_request_plugin_install_discoverable_tools_for_client;
 pub use tool_executor::ToolExecutor;
+pub use tool_executor::ToolExecutorFuture;
 pub use tool_executor::ToolExposure;
 pub use tool_output::JsonToolOutput;
 pub use tool_output::ToolOutput;