Expose execution mode in hook listings (#37538)

## What changed

- Add `executionMode` to `HookMetadata` returned by `hooks/list`, with `sync` as the default for compatibility.
- Propagate each discovered hook's `sync` or `async` mode through the app-server protocol and generated schemas.
- Show the execution mode in the TUI hooks browser.

## Testing

- Cover async and default-sync modes in `hooks/list` tests and TUI snapshots.

GitOrigin-RevId: 9c9f2890798fe417face76c15847616b2dea9db4

codex-rs/app-server-protocol/src/protocol/v2/hook.rs

@@ -28,8 +28,11 @@ v2_enum_from_core!(
 );
 
 v2_enum_from_core!(
+    #[derive(Default)]
     pub enum HookExecutionMode from CoreHookExecutionMode {
-        Sync, Async
+        #[default]
+        Sync,
+        Async
     }
 );
 

codex-rs/app-server-protocol/src/protocol/v2/plugin.rs

@@ -1,5 +1,6 @@
 use super::AppSummary;
 use super::HookEventName;
+use super::HookExecutionMode;
 use super::HookHandlerType;
 use super::HookSource;
 use super::HookTrustStatus;
@@ -524,6 +525,8 @@ pub struct HookMetadata {
     pub key: String,
     pub event_name: HookEventName,
     pub handler_type: HookHandlerType,
+    #[serde(default)]
+    pub execution_mode: HookExecutionMode,
     pub matcher: Option<String>,
     pub command: Option<String>,
     pub timeout_sec: u64,