Make clock reads synchronous and remove client clock requests (#61382)

## What changed

- Make `TimeProvider::current_time` and `CodexThread::current_time` synchronous, with clock reads required to avoid yielding or calling back into the session.
- Remove the experimental `currentTime/read` app-server request and its client-backed clock and sleep polling implementation.
- Require a host-provided `TimeProvider` for external clocks. App-server thread startup now fails when `features.current_time_reminder.clock_source = "external"` has no provider.

## Testing

Add coverage for external provider selection and rejection of app-server thread startup without an external provider.

GitOrigin-RevId: 09479d6eb28453b343dee78b3673092e73ae5d73

codex-rs/exec/src/lib.rs

@@ -2315,15 +2315,6 @@ async fn handle_server_request(
             )
             .await
         }
-        ServerRequest::CurrentTimeRead { request_id, .. } => {
-            reject_server_request(
-                client,
-                request_id,
-                &method,
-                "external current time is not supported in exec mode".to_string(),
-            )
-            .await
-        }
         ServerRequest::ApplyPatchApproval { request_id, params } => {
             reject_server_request(
                 client,