[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/ext/image-generation/src/tool.rs

@@ -78,7 +78,6 @@ struct ImagegenArgs {
     num_last_images_to_include: Option<usize>,
 }
 
-#[async_trait::async_trait]
 impl ToolExecutor<ToolCall> for ImageGenerationTool {
     /// Keeps the tool in the existing image-generation Responses namespace.
     fn tool_name(&self) -> ToolName {
@@ -96,8 +95,8 @@ impl ToolExecutor<ToolCall> for ImageGenerationTool {
     }
 
     /// Executes the selected image operation and returns the completed image result.
-    async fn handle(&self, call: ToolCall) -> Result<Box<dyn ToolOutput>, FunctionCallError> {
-        self.handle_call(call).await
+    fn handle(&self, call: ToolCall) -> codex_extension_api::ToolExecutorFuture<'_> {
+        Box::pin(self.handle_call(call))
     }
 }