[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/tool_executor.rs

@@ -3,6 +3,12 @@ use crate::ToolName;
 use crate::ToolOutput;
 use crate::ToolSearchInfo;
 use crate::ToolSpec;
+use std::future::Future;
+use std::pin::Pin;
+
+/// The boxed future returned by [`ToolExecutor::handle`].
+pub type ToolExecutorFuture<'a> =
+    Pin<Box<dyn Future<Output = Result<Box<dyn ToolOutput>, FunctionCallError>> + Send + 'a>>;
 
 /// Controls where a tool is exposed to the model.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -40,7 +46,6 @@ impl ToolExposure {
 /// Implementations keep the model-visible spec tied to the executable runtime.
 /// Host crates can layer routing, hooks, telemetry, or other orchestration on
 /// top without reopening the spec/runtime split.
-#[async_trait::async_trait]
 pub trait ToolExecutor<Invocation>: Send + Sync {
     /// The concrete tool name handled by this runtime instance.
     fn tool_name(&self) -> ToolName;
@@ -60,8 +65,5 @@ pub trait ToolExecutor<Invocation>: Send + Sync {
         false
     }
 
-    async fn handle(
-        &self,
-        invocation: Invocation,
-    ) -> Result<Box<dyn ToolOutput>, FunctionCallError>;
+    fn handle(&self, invocation: Invocation) -> ToolExecutorFuture<'_>;
 }