Remove async-trait from extension contributors (#27383)

## Why

Extension contributors are registered behind `dyn Trait` objects, so
native `async fn`/RPITIT methods would make these traits
non-object-safe. Spell out the boxed, `Send` future contract directly so
`extension-api` no longer needs `async-trait` while retaining the
existing runtime model.

## What changed

- add a shared `ExtensionFuture` alias and use it for asynchronous
contributor methods
- migrate production and test implementations to return `Box::pin(async
move { ... })`
- remove `async-trait` dependencies where they are no longer used,
keeping it dev-only where unrelated test executors still require it

## Behavior

No behavior change is intended. Contributor futures remain boxed,
`Send`, dynamically dispatched, and lazily executed; cancellation and
callback ordering stay unchanged.

## Testing

- `just test -p codex-extension-api` (11 passed)
- affected extension crates (64 passed)
- targeted `codex-core` contributor tests (14 passed)
- `just fmt`
- `just bazel-lock-update`
- `just bazel-lock-check`

A broad local `codex-core` run compiled successfully but encountered
unrelated sandbox and missing test-binary fixture failures; CI will run
the full checks.

codex-rs/core/src/tools/handlers/extension_tools.rs

@@ -388,16 +388,17 @@ mod tests {
 
     struct RecordExtensionTurnItemContributor;
 
-    #[async_trait::async_trait]
     impl TurnItemContributor for RecordExtensionTurnItemContributor {
-        async fn contribute(
-            &self,
-            _thread_store: &ExtensionData,
-            turn_store: &ExtensionData,
-            _item: &mut TurnItem,
-        ) -> Result<(), String> {
-            turn_store.insert(ExtensionTurnItemContributorRan);
-            Ok(())
+        fn contribute<'a>(
+            &'a self,
+            _thread_store: &'a ExtensionData,
+            turn_store: &'a ExtensionData,
+            _item: &'a mut TurnItem,
+        ) -> codex_extension_api::ExtensionFuture<'a, Result<(), String>> {
+            Box::pin(async move {
+                turn_store.insert(ExtensionTurnItemContributorRan);
+                Ok(())
+            })
         }
     }