acp_thread: Mark in progress plan items as pending (#52196)

## Context

We were seeing some off by one errors because we weren't counting in
progress as either pending or completed.

Release Notes:

- acp: Fix plan stats showing all tasks as done when items were still in
progress.

crates/acp_thread/src/acp_thread.rs

@@ -924,6 +924,7 @@ impl Plan {
                 }
                 acp::PlanEntryStatus::InProgress => {
                     stats.in_progress_entry = stats.in_progress_entry.or(Some(entry));
+                    stats.pending += 1;
                 }
                 acp::PlanEntryStatus::Completed => {
                     stats.completed += 1;