[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/core/src/tools/router_tests.rs

@@ -44,7 +44,6 @@ impl codex_extension_api::ToolContributor for ExtensionEchoContributor {
 
 struct ExtensionEchoExecutor;
 
-#[async_trait::async_trait]
 impl ToolExecutor<ExtensionToolCall> for ExtensionEchoExecutor {
     fn tool_name(&self) -> ToolName {
         ToolName::namespaced("extension/", "echo")
@@ -73,11 +72,8 @@ impl ToolExecutor<ExtensionToolCall> for ExtensionEchoExecutor {
         })
     }
 
-    async fn handle(
-        &self,
-        call: ExtensionToolCall,
-    ) -> Result<Box<dyn codex_tools::ToolOutput>, codex_tools::FunctionCallError> {
-        self.handle_call(call).await
+    fn handle(&self, call: ExtensionToolCall) -> codex_tools::ToolExecutorFuture<'_> {
+        Box::pin(self.handle_call(call))
     }
 }
 
@@ -93,7 +89,7 @@ impl ExtensionEchoExecutor {
             "callId": call.call_id,
             "conversationHistory": call.conversation_history.items(),
             "ok": true,
-        }))))
+        }))) as Box<dyn codex_tools::ToolOutput>)
     }
 }