Fix PostgreSQL deadlock with Close Statement flood exceeding threshold_resultset_size

Bug Description:
ProxySQL would deadlock when processing extended query frames where:
1. Many Close Statement messages accumulate responses in PSarrayOUT
2. Total response size exceeds pgsql-threshold_resultset_size
3. A backend operation (Describe/Execute) follows in the same frame

Root Cause:
- Close Statement operations are handled locally by ProxySQL (no backend routing)
- Their CloseComplete responses accumulate in PSarrayOUT
- When threshold_resultset_size is exceeded, ProxySQL stops reading from backend
- Subsequent backend operations (Describe/Execute) need backend responses to complete
- This creates a deadlock: ProxySQL won't read, backend operation can't complete
- Extended query frame never finishes, query times out

The Fix:
When PSarrayOUT exceeds threshold_resultset_size and a backend operation is pending,
ProxySQL now flushes all accumulated data in PSarrayOUT to the client first, then
continues processing backend operations. This breaks the deadlock by clearing the
buffer before attempting to read more data from the backend.

lib/PgSQL_Session.cpp

@@ -3094,7 +3094,17 @@ handler_again:
 						if (myconn->query_result && myconn->query_result->get_resultset_size() > (unsigned int)pgsql_thread___threshold_resultset_size) {
 							myconn->query_result->get_resultset(client_myds->PSarrayOUT);
 						} else {
-							in_pending_state = true;
+
+							if (processing_extended_query && client_myds && mirror == false) {
+								const unsigned int buffered_data = client_myds->PSarrayOUT->len * PGSQL_RESULTSET_BUFLEN;
+								if (buffered_data > overflow_safe_multiply<4, unsigned int>(pgsql_thread___threshold_resultset_size)) {
+									// Don't enter pending state when PSarrayOUT exceeds threshold. This allows ProxySQL
+									// to flush accumulated data to the client before attempting to read backend responses.
+									// Prevents deadlock. Issue#5300
+								} else {
+									in_pending_state = true;
+								}
+							}
 						}
 						break;
 						// rc==2 : a multi-resultset (or multi statement) was detected, and the current statement is completed