[codex] Outline ToolExecutor handler bodies (#27299)

## 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.

For ease of reviewing, this is a prefactor to extract trait method
implementations to inherent methods. This will prevent changing
indentation from creating a huge diff.

## What

Outlined existing `ToolExecutor::handle` bodies into inherent async
`handle_call` methods across core and extension tool handlers.

The trait methods still use `async_trait` and now delegate to
`self.handle_call(...).await`; handler behavior is unchanged.

codex-rs/core/src/tools/router_tests.rs

@@ -76,6 +76,15 @@ 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
+    }
+}
+
+impl ExtensionEchoExecutor {
+    async fn handle_call(
+        &self,
+        call: ExtensionToolCall,
     ) -> Result<Box<dyn codex_tools::ToolOutput>, codex_tools::FunctionCallError> {
         let arguments: serde_json::Value =
             serde_json::from_str(call.function_arguments()?).expect("test arguments should parse");